Add reference file for PWG raster generation test

This file is not used in the test itself, but is referenced in a comment as
a reference "good" file in case the test fails (the test itself uses a hardcoded
SHA1 hash for speed/simplicity).

BUG=318373
R=gene@chromium.org

Review URL: https://codereview.chromium.org/64553007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235200 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235200