Update test runner code since showContextMenu is moving from WebViewClient to WebFrameClient.

See the build error in patchset 3 of this Blink change: https://codereview.chromium.org/183693004/

BUG=304341
R=nasko@chromium.org

Review URL: https://codereview.chromium.org/184793002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254192 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254192