replace TestPixelRef with bitmap.allocPixels() -- removing unneeded subclass of pixelref

BUG=

Review URL: https://codereview.chromium.org/136283004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244926 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244926