Remove the enable-d3d11 flag so that d3d11 is enabled by default.

BUG=346394
R=kbr@chromium.org

Review URL: https://codereview.chromium.org/177003009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254571 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254571