GitLab

* ShareableFileReference is not thread-safe, we should make sure we only handle it on IO thread
* FileUtil implementation no longer needs to directly handle ShareableFileReference

BUG=140508


Review URL: https://chromiumcodereview.appspot.com/10834167

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@149992 0039d316-1c4b-4281-b951-d872f2087c98