Enable SSE2 compilation for ChromeOS.
In practice the <video> tag needs this for faster YUV conversion/filtering.
BUG=19113
TEST=Test youtube html5 with chromeos (not chromiumos).  It should not crash, and it should be faster.

Review URL: http://codereview.chromium.org/1553035

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@44740 0039d316-1c4b-4281-b951-d872f2087c98