Fix 2 NPEs in the Browser.

Fix 2 NPEs in the Browser.

The first was from freeMemory getting a tab back that had already been freed.
This is a really hard case to reproduce but it appears that it can happen. So
just check for a null mMainView before trying to free the tab.

The second was a null mTabOverview during the onAnimationStart method when
animating to the tab overview. I think this is caused by message delays not
being accurate enough to ensure the overview has been created. A check for null
fixes the problem for now but I am working on a better solution to ensure
animation message order.