Texture provider to feed data to the impl side thread for webview compositing.

Texture provider to feed data to the impl side thread for webview compositing.
This is still awaiting the webkit patch with the texture provider interface,
but I am putting this up for comments in the mean time.

This is the renderer side machinery to make compositing work,
but it's not turned on yet because other pieces are still missing.
That said, this is a part of a working prototype and I want to split
it up into afew smaller patches to make reviewing it easier.  

BUG=143429