Remove old about scheme URL constants.

These are no longer needed after chrome frame's deletion.
Use the equivalent chrome scheme URL constants instead.

BUG=316496
TEST=NONE
R=pkasting@chromium.org
TBR=bauerb@chromium.org

Review URL: https://codereview.chromium.org/137793017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246199 0039d316-1c4b-4281-b951-d872f2087c98
Rename the UseRealGLContexts() to EnablePixelOutput().

Now we always use a real gl context, this function name is wrong. Also
add a command line flag to force pixel output to be enabled for visual
debugging, as --enable-pixel-output-in-tests.

A number of tests set UseRealGLContexts() for reasons that don't exist
anymore so clean these up to try only calling EnablePixelOutput() from
tests that actually compare the output pixels as a criteria for passing.

R=piman, sky
BUG=270918

Review URL: https://codereview.chromium.org/143893008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246200 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246200