Coverity: Check return of TabProxy::GoForward, GoBack, etc
Coverity: Check return of TabProxy::GoForward, GoBack, etc

CID=5709,5708,5706,5705,5704,5703,5702,5701,5700
BUG=none
TEST=ran the tests

Review URL: http://codereview.chromium.org/256067

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@28340 0039d316-1c4b-4281-b951-d872f2087c98

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