Add basic support to dump pixel results

BUG=111316
TEST="webkit/tools/layout_tests/run_webkit_tests.py --chromium --debug --driver-name=content_shell --additional-drt-flag=--dump-render-tree css3/css3-modsel-33.html" should pass

Review URL: https://chromiumcodereview.appspot.com/10656044

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@144436 0039d316-1c4b-4281-b951-d872f2087c98
6 files changed