Re-enable context-attribute-preserve-drawing-buffer.html on Win/Intel

This might still fail.  Will revert if that's the case.

BUG=322770
TEST=gpu win intel bots
TBR=kbr@chromium.org

Review URL: https://codereview.chromium.org/155823003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249827 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249827