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

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 //headless.

This CL was uploaded by git cl split.

R=dvallet@chromium.org

Bug: 837148
Change-Id: I1b20f722f974410fa64cab3bd4c2fc56732bbe42
Reviewed-on: https://chromium-review.googlesource.com/1031199Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555392}

headless/lib/headless_web_contents_browsertest.cc

@@ -71,7 +71,7 @@ class MockHeadlessBrowserContextObserver
                void(HeadlessWebContents*, HeadlessWebContents*));
 
   MockHeadlessBrowserContextObserver() = default;
-  virtual ~MockHeadlessBrowserContextObserver() = default;
+  ~MockHeadlessBrowserContextObserver() override = default;
 
   HeadlessWebContents* last_parent;
   HeadlessWebContents* last_child;