GitLab

This CL do followings to ensure to update thumbnails after the app saves images.

* Add '?nocache=xxx' to FileEntry's URL to ignore browser caches.
* Stop to use drive thumbnails for files that are on the drive but cached
  locally. These files may be edited locally and be out of sync with the local
  cache.
* Specify the current time as modificationTime of the metadata that is
  dispatched when the edited file is saved.

BUG=378648
TEST=manually

Review URL: https://codereview.chromium.org/310433004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@274506 0039d316-1c4b-4281-b951-d872f2087c98