Restore keyboard shortcut for expand all diff context

In Ib3fe44d502a the keyboard shortcuts system was redesigned, but the
binding for Shift+x shortcut for expand all diffs was lost. Restore
this binding and confirm that it is now listed in the help dialog and
works as expected.

Bug: Issue 13054
Change-Id: Ia122804b894972ae892670bf8f7d8a48698d859f
diff --git a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
index 072708e..ae64c3a 100644
--- a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
+++ b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
@@ -439,6 +439,7 @@
       [this.Shortcut.UP_TO_DASHBOARD]: '_handleUpToDashboard',
       [this.Shortcut.EXPAND_ALL_MESSAGES]: '_handleExpandAllMessages',
       [this.Shortcut.COLLAPSE_ALL_MESSAGES]: '_handleCollapseAllMessages',
+      [this.Shortcut.EXPAND_ALL_DIFF_CONTEXT]: '_expandAllDiffs',
       [this.Shortcut.OPEN_DIFF_PREFS]: '_handleOpenDiffPrefsShortcut',
       [this.Shortcut.EDIT_TOPIC]: '_handleEditTopic',
       [this.Shortcut.DIFF_AGAINST_BASE]: '_handleDiffAgainstBase',