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This change adds the apple software renderer as an option on Chrome/Mac.  It's enabled by passing --use-gl=apple on the command line.  It also removes the default pbuffer creation on Mac since it's not required to create GL contexts and is incompatible with the generic float renderer.

BUG=106584
TEST=--use-gl=apple on chrome/mac cmdline
Review URL: http://codereview.chromium.org/8698008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@113436 0039d316-1c4b-4281-b951-d872f2087c98