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

.gitignore

@@ -129,6 +129,8 @@ v8.log
 /cloud_print/cloud_print_version_resources.xml
 /components/variations.xml
 /content/content_unittests_run.xml
+/content/test/data/gpu/generated/
+/content/test/data/gpu/gpu_reference/
 /content/test/data/layout_tests/
 /content/test/data/plugin/
 /data