Remove win mock theme engine code.

Now that Aura is required for Windows, we can safely delete the non-Aura
mock theme code and simplify the logic for the other cases.

R=jam@chromium.org
BUG=330735

Review URL: https://codereview.chromium.org/139903015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247799 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247799