Include the default implementation of WebThemeEngine.h in webthemeengine_impl_android.h

Rather than using the (now-obsolete) version in platform/android. Once
this lands, we can delete the android stub.

R=jamesr@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/120313003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245589 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245589