GitLab

GetDownloadDataCallback parameter in GDataFileSystem::GetFileByPath()
was optional hence a null callback was passed in many places, and
the null callback was posted to a message loop without checking if
it's null or not.

BUG=chromium-os:30198
TEST=double clicking on an image file on gdata; copy-pasting from gdata to Downloads; both don't crash

Review URL: https://chromiumcodereview.appspot.com/10288006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@134808 0039d316-1c4b-4281-b951-d872f2087c98