Run accessibility unit tests through base test suite.

Using gtest.gyp:gtest_main is very uncommon in chromium tree, there are
only three instances. By running it through base:run_all_unittests
should be more robust.

BUG=None
TEST=accessibility_unittests
R=dmazzoni@chromium.org

Review URL: https://codereview.chromium.org/110983002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239739 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239739