gpu: Fix reference image writing in pixel tests

The bitmap.WriteFile method was renamed to WritePngFile.

BUG=323813
TBR=kbr@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/104463002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238693 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238693