Revert r250851 - "Update all the client code to use a single WebThemeEngine.h header."

This changes depends on r166988, and that hasn't actually rolled yet :(.

TBR=jamesr@chromium.org
BUG=327471

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250856 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250856