Revert 149576 - gdata: Make WeakPtrFactory the last parameter

Revert 149576 - gdata: Make WeakPtrFactory the last parameter

Broke RemoteFileSystemExtensionApiTest.RemoteMountPoint

Remove WeakPtr members in favor of WeakPtrFactory::GetWeakPtr()
Along the way, copy derat's comment from ContactDatabase on this matter.

Wez on [chromium-dev] Weak ptrs for derived types?

You need to be similarly careful to have WeakPtrFactory appear after any
fields that its WeakPtrs may be used to access if you follow that pattern
instead.

BUG=none
TEST=none

Review URL: https://chromiumcodereview.appspot.com/10837061

TBR=satorux@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=149582