[Extensions] Fix a11y announcement when shortcut is set.

On chrome://extensions/shortcuts, when shortcut is set, a11y
announcement is not correct, since it announces "Shortcut is not set".
This CL fixes this bug, i.e., when shortcut is set, it announces
"Shortcut set", followed by the shortcut.

a shortcut, and navigate back to re-read the set shortcut.

Bug: 1031007
Test: Enable screenreading, navigate to chrome://extensions/, set
Change-Id: Ib128b637222ff3e0f92742781e75d8320edb345a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2625749
Commit-Queue: Ghazale Hosseinabadi <ghazale@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843383}

chrome/browser/resources/extensions/shortcut_input.js

@@ -256,7 +256,8 @@ Polymer({
    */
   computePlaceholder_() {
     if (this.readonly_) {
-      return this.i18n('shortcutNotSet');
+      return this.shortcut ? this.i18n('shortcutSet', this.computeText_()) :
+                             this.i18n('shortcutNotSet');
     }
     return this.i18n('shortcutTypeAShortcut');
   },