Fix crash when quickly dragging a new tab.

Fix crash when quickly dragging a new tab.

The root cause is r185551, which changed the semantics of
scoped_ptr::reset(). This code depended on the fact that a get() during
a reset() would return the original value of the stored pointer, but
after r185551, it returns the value that it is being reset to. This
caused the code to stop an animation and the code to end a drag to
conflict, since the animation code would sometimes delete a TabGtk that
was still being dragged.

BUG=228918
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=202184

[Elliot Glaysher, 2013-05-24 18:29:56]

lgtm

[commit-bot: I haz the power, 2013-05-24 18:40:32]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/dcheng@chromium.org/16035005/1

[commit-bot: I haz the power, 2013-05-24 21:49:43]

Change committed as 202184