[api-minor] Add basic support for PageLayout in the API and the viewer

Please see the specification, https://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/PDF32000_2008.pdf#G6.2393749, and refer to the inline comments for additional details.

src/core/obj.js

@@ -376,6 +376,27 @@ class Catalog {
     return pageLabels;
   }
 
+  get pageLayout() {
+    const obj = this.catDict.get('PageLayout');
+    // Purposely use a non-standard default value, rather than 'SinglePage', to
+    // allow differentiating between `undefined` and /SinglePage since that does
+    // affect the Scroll mode (continuous/non-continuous) used in Adobe Reader.
+    let pageLayout = '';
+
+    if (isName(obj)) {
+      switch (obj.name) {
+        case 'SinglePage':
+        case 'OneColumn':
+        case 'TwoColumnLeft':
+        case 'TwoColumnRight':
+        case 'TwoPageLeft':
+        case 'TwoPageRight':
+          pageLayout = obj.name;
+      }
+    }
+    return shadow(this, 'pageLayout', pageLayout);
+  }
+
   get pageMode() {
     const obj = this.catDict.get('PageMode');
     let pageMode = 'UseNone'; // Default value.

src/core/worker.js

@@ -517,6 +517,10 @@ var WorkerMessageHandler = {
       }
     );
 
+    handler.on('GetPageLayout', function wphSetupGetPageLayout(data) {
+      return pdfManager.ensureCatalog('pageLayout');
+    });
+
     handler.on('GetPageMode', function wphSetupGetPageMode(data) {
       return pdfManager.ensureCatalog('pageMode');
     });

src/display/api.js

@@ -657,6 +657,14 @@ class PDFDocumentProxy {
     return this._transport.getPageLabels();
   }
 
+  /**
+   * @return {Promise} A promise that is resolved with a {string} containing
+   *   the page layout name.
+   */
+  getPageLayout() {
+    return this._transport.getPageLayout();
+  }
+
   /**
    * @return {Promise} A promise that is resolved with a {string} containing
    *   the page mode name.
@@ -2214,6 +2222,10 @@ class WorkerTransport {
     return this.messageHandler.sendWithPromise('GetPageLabels', null);
   }
 
+  getPageLayout() {
+    return this.messageHandler.sendWithPromise('GetPageLayout', null);
+  }
+
   getPageMode() {
     return this.messageHandler.sendWithPromise('GetPageMode', null);
   }