Fixes two bugs in NavigationController:
Fixes two bugs in NavigationController:

. TabContentsWasDestroyed wasn't removing from the collector map,
  resulting.
. The collector map wasn't cleared in Destroy when it should have
  been.

This was the cause of random ui failures.

BUG=1324555
TEST=none


git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@870 0039d316-1c4b-4281-b951-d872f2087c98

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