Add support for over-riding config attribute values.

GLSurfaceEGL chooses a EGL frame buffer configuration that match
attributes specified in attrib list. This CL adds support for Ozone
implementations to provide values for these attributes as supported
by the underlying window system.

BUG=

Review URL: https://codereview.chromium.org/23850008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226925 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226925