[CRX] Make textLayerMode pref visible and add migration logic

In a1cfa5f4d7, the textLayerMode
preference was introduced, to replace the disableTextLayer and
enhanceTextSelection preferences.

As a result, the text selection preference was no longer visible
in Chrome (because preferences are only rendered by default for
boolean preferences, not for enumerations).

This commit adds the necessary bits to
extensions/chromium/options/options.{html,js}
so that the textLayerMode preference can be changed again.

Also, migration logic has been added to move over preferences
from the old to the new names:
- In web/chromecom.js, the logic is added to translate
  preferences that were set by an administrator (it is read-only,
  so this layer is unavoidable).
- In extensions/chromium/options/migration.js, similar logic is
  added, except in this case the preference storage is writable,
  so this migration logic happens only once.

The "enhanced text selection" mode is still experimental, so it
has been marked as experimental to signal that there may be bugs.
The list of tasks that block promotion to stable is at #7584.

extensions/chromium/options/options.js

@@ -81,6 +81,8 @@ Promise.all([
       renderPreference = renderSidebarViewOnLoad(prefSchema.title);
     } else if (prefName === 'cursorToolOnLoad') {
       renderPreference = renderCursorToolOnLoad(prefSchema.title);
+    } else if (prefName === 'textLayerMode') {
+      renderPreference = renderTextLayerMode(prefSchema.title);
     } else if (prefName === 'externalLinkTarget') {
       renderPreference = renderExternalLinkTarget(prefSchema.title);
     } else {
@@ -217,6 +219,23 @@ function renderCursorToolOnLoad(shortDescription) {
   return renderPreference;
 }
 
+function renderTextLayerMode(shortDescription) {
+  var wrapper = importTemplate('textLayerMode-template');
+  var select = wrapper.querySelector('select');
+  select.onchange = function() {
+    storageArea.set({
+      textLayerMode: parseInt(this.value),
+    });
+  };
+  wrapper.querySelector('span').textContent = shortDescription;
+  document.getElementById('settings-boxes').appendChild(wrapper);
+
+  function renderPreference(value) {
+    select.value = value;
+  }
+  return renderPreference;
+}
+
 function renderExternalLinkTarget(shortDescription) {
   var wrapper = importTemplate('externalLinkTarget-template');
   var select = wrapper.querySelector('select');