gpu: Add more context labelling.

Make sure names are unique in different renderers, and add
one for offscreen renderer context (used for filters).

The offscreen context is created lazily, so the label is also
applied lazily.

BUG=242999,233101

Review URL: https://chromiumcodereview.appspot.com/17350003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@209284 0039d316-1c4b-4281-b951-d872f2087c98
4 files changed