Feedback: Fix the broken image screenshot

The spec suggests showing a broken image icon when and <img> element
has an alt text attribute but no src attribute. To avoid this, we
set the alt text only when we set the src of the img element.

BUG=773985
TEST=manual

Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I3884e736a8a7b0e3fa40ea483882d20916a81a00
Reviewed-on: https://chromium-review.googlesource.com/727107Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510098}

chrome/browser/resources/feedback/html/default.html

@@ -72,7 +72,7 @@
     <div id="screenshot-container" class="checkbox-field-container">
       <input id="screenshot-checkbox" type="checkbox" aria-labelledby="screenshot-label">
       <label id="screenshot-label" i18n-content="screenshot"></label>
-      <img id="screenshot-image" alt="screenshot">
+      <img id="screenshot-image">
     </div>
     <!-- System Information -->
     <div class="checkbox-field-container">

chrome/browser/resources/feedback/js/feedback.js

@@ -348,7 +348,11 @@ function initialize() {
         chrome.app.window.current().show();
 
         var screenshotDataUrl = screenshotCanvas.toDataURL('image/png');
+
+        // Only set the alt text when the src url is available, otherwise we'd
+        // get a broken image picture instead. crbug.com/773985.
         $('screenshot-image').src = screenshotDataUrl;
+        $('screenshot-image').alt = 'screenshot';
         $('screenshot-image')
             .classList.toggle(
                 'wide-screen',