gdata: Reduce use of GetWeakPtrForCurrentThread()
gdata: Reduce use of GetWeakPtrForCurrentThread()

In some functions, we know that we are on UI thread, so we can
directly use ui_weak_ptr_.

BUG=chromium-os:28503
TEST=none

Review URL: https://chromiumcodereview.appspot.com/10169041

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@133811 0039d316-1c4b-4281-b951-d872f2087c98

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