content: Do not call empty methods of WebScriptController.

enableV8SingleThreadMode() and flushConsoleMessages() do nothing in
their Blink implementation, so we don't need to call them as they won't
do anything at all.

BUG=None
TEST=content_shell, content_unittests
R=abarth@chromium.org, jochen@chromium.org

Review URL: https://codereview.chromium.org/169173003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251716 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251716