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GDataFileSystem was a ref counted object, but deleted by
ProfileKeyedServiceFactory with 'delete'. This was horribly
wrong, and resulted in a crash at deletion of GDataFileSystem
that happened at browser shutdown time.

The fix is to make GDataFileSystem non-ref counted.
WeakPtr is used to create callbacks.

Along the way, replaced kGDataAPICallThread with BrowserThread::UI
in gdata.cc. TokenFetcher requires callers to be on UI thread.

BUG=chromium-os:27121
TEST=Run chrome on debug build, open the file manager, show gdata files, Ctrl-C to exit. No crash from GDataFileSystem

Review URL: https://chromiumcodereview.appspot.com/9564002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@124554 0039d316-1c4b-4281-b951-d872f2087c98