Changes x11_util::GetRenderVisualFormat to keep a cache of render

picts. This way we can support different visuals while still caching
the value.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/210021

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@26615 0039d316-1c4b-4281-b951-d872f2087c98