Chrome style plugin: enable virtual specifier checks for Gmock objects in //mojo.

Previously, the plugin completely skipped virtual specifier checks on
Gmock objects since it wasn't smart enough to filter out warnings caused
by Gmock macros. In preparation for making the plugin smarter, this CL
fixes existing virtual specifier warnings in //mojo.

This CL was uploaded by git cl split.

R=jam@chromium.org

Bug: 837148
Change-Id: Iface36adbfe57737c6601cc17d528e04254ee3df
Reviewed-on: https://chromium-review.googlesource.com/1031186Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554806}

mojo/edk/system/channel_unittest.cc

@@ -37,7 +37,7 @@ class TestChannel : public Channel {
   MOCK_METHOD0(ShutDownImpl, void());
   MOCK_METHOD0(LeakHandle, void());
 
-  void Write(MessagePtr message) {}
+  void Write(MessagePtr message) override {}
 
  protected:
   ~TestChannel() override {}