Add GPU pixel test reference image directories to svn:ignore


git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238713 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238713
Add GPU pixel test reference image directories to .gitignore

This is needed to avoid clobbering the reference images during a gclient
revert.

NOTRY=true

Review URL: https://codereview.chromium.org/94093004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238714 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238714