Force use_ash to 0 if use_aura is 0.

Also remove old Windows gyp workaround to enable Ash if use_aura on Windows (as it is now on by default).

BUG=Using use_aura=0 on Windows doesn't compile as it tries to build Ash.

Review URL: https://codereview.chromium.org/25665008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227160 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227160