Highly improved Chrome extension

Full list feature changes in this commit:
- Support for iframes
- Switched to content-type (MIME) detection instead of hard-coding a
  case-sensitive check for the .PDF extension
- The PDF's original URL is visible in the omnibox
- Support for incognito mode

Note: PDF viewer is disabled for the file:// + incognito
combination, because it's currently impossible to get the combination
to work.

See https://github.com/mozilla/pdf.js/pull/3017#issuecomment-15693432

extensions/chrome/pdfHandler.js

@@ -23,25 +23,94 @@ function isPdfDownloadable(details) {
   return details.url.indexOf('pdfjs.action=download') >= 0;
 }
 
-chrome.webRequest.onBeforeRequest.addListener(
+function insertPDFJSForTab(tabId, url) {
+  chrome.tabs.executeScript(tabId, {
+    file: 'insertviewer.js',
+    allFrames: true,
+    runAt: 'document_start'
+  }, function() {
+    chrome.tabs.sendMessage(tabId, {
+      type: 'showPDFViewer',
+      url: url
+    });
+  });
+}
+function activatePDFJSForTab(tabId, url) {
+  chrome.tabs.onUpdated.addListener(function listener(_tabId) {
+    if (tabId === _tabId) {
+      insertPDFJSForTab(tabId, url);
+      chrome.tabs.onUpdated.removeListener(listener);
+    }
+  });
+}
+
+chrome.webRequest.onHeadersReceived.addListener(
   function(details) {
-    if (isPdfDownloadable(details))
+    // Check if the response is a PDF file
+    var isPDF = false;
+    var headers = details.responseHeaders;
+    var header, i;
+    var cdHeader;
+    if (!headers)
+      return;
+    for (i=0; i<headers.length; ++i) {
+      header = headers[i];
+      if (header.name.toLowerCase() == 'content-type') {
+        var headerValue = header.value.toLowerCase().split(';',1)[0].trim();
+        isPDF = headerValue === 'application/pdf' ||
+                headerValue === 'application/octet-stream' &&
+                details.url.toLowerCase().indexOf('.pdf') > 0;
+        break;
+      }
+    }
+    if (!isPDF)
       return;
 
-    var viewerPage = 'content/web/viewer.html';
-    var url = chrome.extension.getURL(viewerPage) +
-      '?file=' + encodeURIComponent(details.url);
-    return { redirectUrl: url };
+    if (isPdfDownloadable(details)) {
+      // Force download by ensuring that Content-Disposition: attachment is set
+      if (!cdHeader) {
+        for (; i<headers.length; ++i) {
+          header = headers[i];
+          if (header.name.toLowerCase() == 'content-disposition') {
+            cdHeader = header;
+            break;
+          }
+        }
+      }
+      if (!cdHeader) {
+        cdHeader = {name: 'Content-Disposition', value: ''};
+        headers.push(cdHeader);
+      }
+      if (cdHeader.value.toLowerCase().indexOf('attachment') === -1) {
+        cdHeader.value = 'attachment' + cdHeader.value.replace(/^[^;]+/i, '');
+        return {
+          responseHeaders: headers
+        };
+      }
+      return;
+    }
+
+    // Replace frame's content with the PDF viewer
+    // This approach maintains the friendly URL in the 
+    // location bar
+    activatePDFJSForTab(details.tabId, details.url);
+
+    return {
+      responseHeaders: [
+        // Set Cache-Control header to avoid downloading a file twice
+        {name:'Cache-Control',value:'max-age=600'},
+        // Temporary render response as XHTML.
+        // Since PDFs are never valid XHTML, the garbage is not going to be
+        // rendered. insertviewer.js will quickly replace the document with
+        // the PDF.js viewer.
+        {name:'Content-Type',value:'application/xhtml+xml; charset=US-ASCII'},
+      ]
+    };
   },
   {
     urls: [
-      'http://*/*.pdf',
-      'https://*/*.pdf',
-      'file://*/*.pdf',
-      'http://*/*.PDF',
-      'https://*/*.PDF',
-      'file://*/*.PDF'
+      '<all_urls>'
     ],
-    types: ['main_frame']
+    types: ['main_frame', 'sub_frame']
   },
-  ['blocking']);
+  ['blocking','responseHeaders']);