Fix "Behavior must be global names..." error

This error is one of many showing up when you set
`compilation_mode="SIMPLE"`, so fixing it is a prerequisite for
turning on compilation:

ERROR - Behaviors must be global names or qualified names that are declared as object literals or array literals of other valid Behaviors.
      Gerrit.KeyboardShortcutBehavior,
      ^
  ProTip: "JSC_POLYMER_UNQUALIFIED_BEHAVIOR" can be added to the `suppress` attribute of:
  //polygerrit-ui/app:polygerrit_ui_closure_lib

Also adding a new entry point for gr-diff because we need to add the
`let Gerrit = ...` line somewhere at the root that is not imported
elsewhere.

Change-Id: I47a487e31d38855978e6a449aab5978edc050d6d
diff --git a/polygerrit-ui/app/elements/gr-app.html b/polygerrit-ui/app/elements/gr-app.html
index e7bd965..758add8 100644
--- a/polygerrit-ui/app/elements/gr-app.html
+++ b/polygerrit-ui/app/elements/gr-app.html
@@ -26,6 +26,9 @@
       passiveTouchGestures: true,
     };
   }
+  // Needed for JSCompiler to understand it's global.
+  // eslint-disable-next-line no-unused-vars, prefer-const
+  let Gerrit = window.Gerrit || {};
 </script>
 
 <link rel="import" href="../bower_components/polymer/polymer.html">
diff --git a/polygerrit-ui/app/embed/embed.html b/polygerrit-ui/app/embed/embed.html
index 9fb5c23..647f602 100644
--- a/polygerrit-ui/app/embed/embed.html
+++ b/polygerrit-ui/app/embed/embed.html
@@ -14,6 +14,11 @@
 See the License for the specific language governing permissions and
 limitations under the License.
 -->
+<script>
+  // Needed for JSCompiler to understand it's global.
+  // eslint-disable-next-line no-unused-vars, prefer-const
+  let Gerrit = window.Gerrit || {};
+</script>
 <link rel="import" href="../bower_components/polymer/polymer.html">
 <link rel="import" href="../elements/change/gr-change-view/gr-change-view.html">
 <link rel="import" href="../elements/core/gr-search-bar/gr-search-bar.html">
diff --git a/polygerrit-ui/app/gr-diff/gr-diff-root.html b/polygerrit-ui/app/gr-diff/gr-diff-root.html
new file mode 100644
index 0000000..557f48b
--- /dev/null
+++ b/polygerrit-ui/app/gr-diff/gr-diff-root.html
@@ -0,0 +1,6 @@
+<script>
+  // Needed for JSCompiler to understand it's global.
+  // eslint-disable-next-line no-unused-vars, prefer-const
+  let Gerrit = window.Gerrit || {};
+</script>
+<link rel="import" href="../elements/diff/gr-diff/gr-diff.html">
\ No newline at end of file