PolyGerrit diff comment tweaks

+ Use buttons instead of links so that the space bar can be used
  to activate them as well as enter.
+ Allow 'esc' as a shortcut to cancel.
+ Add 2px padding to the textarea so that it matches the mirror
  it uses internally and resizes correctly.

Bug: Issue 3780
Change-Id: Iad9dc96274af6946f3632be53b106ef80f7ba6ca
diff --git a/polygerrit-ui/app/test/gr-diff-comment-test.html b/polygerrit-ui/app/test/gr-diff-comment-test.html
index 050a1a0..d6e46bd 100644
--- a/polygerrit-ui/app/test/gr-diff-comment-test.html
+++ b/polygerrit-ui/app/test/gr-diff-comment-test.html
@@ -190,12 +190,14 @@
       var numDiscardEvents = 0;
       element.addEventListener('discard', function(e) {
         numDiscardEvents++;
-        if (numDiscardEvents == 2) {
+        if (numDiscardEvents == 3) {
           done();
         }
       });
       MockInteractions.tap(element.$$('.cancel'));
       MockInteractions.tap(element.$$('.discard'));
+      MockInteractions.pressAndReleaseKeyOn(
+          element.$.editTextarea.textarea, 27);  // 'esc'
     });
 
     test('draft saving/editing', function(done) {