Set MinimumRequiredVersion for XP support on x86 builds.

BUG=297971
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/140623005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249318 0039d316-1c4b-4281-b951-d872f2087c98
Do not build the Mock WebThemeEngine implementation on the Mac.

Since it doesn't actually work.

R=jamesr@chromium.org, piman@chromium.org
BUG=327471

Review URL: https://codereview.chromium.org/155443002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249319 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249319