Mac tabpose: Add thumbnails
Most things actually work.
Missing from this CL:
* Reloading thumbnails for tabs that change. This is required to show non-white thumbnails for thumbnails that are still waiting on the net when tabpose is opened.
* Showing infobars / bookmark bar in the thumbnail
* Showing accelerated surfaces (youtube videos on 10.6, compositor on 10.6)
TEST=Enable tabpose. Should see thumbnails for all tabs (some loaded after a delay). Thumbnails should animate in correctly even if a tab has info bars, a detached NTP, or docked devtools. Tabs that haven't been frontmost since the window was last resized should look good. Opening many tabs and then immediately jumping into expose shouldn't crash.