Files.app: Replace the Google Drive label in the middle pane with My Drive.

This should be match with the web UI.

BUG=325664
TEST=Show Files.app.

Review URL: https://codereview.chromium.org/103943007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239224 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239224