Make it possible to draw non-mock controls in layout tests

This CL add testRunner.setUseMockTheme(boolean) to the
--dump-render-tree environemnt of content shell.

The initial value of the flag is 'true.'.

BUG=338232
TEST=none; will add layout tests using it.

Review URL: https://codereview.chromium.org/148133002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247433 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247433