GitLab

For future use with favicons, remove reference to "thumbnail". There is no functionality change in this CL, just a rename.

BUG=388747
TEST=ImageManager*

Review URL: https://codereview.chromium.org/445753004

Cr-Commit-Position: refs/heads/master@{#288260}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288260 0039d316-1c4b-4281-b951-d872f2087c98