Support partial invalidation of tiles based on webkit's inval rect.

Support partial invalidation of tiles based on webkit's inval rect.

This CL causes the selective repainting of BaseTiles (and their
associated GL textures) using the information provided by Webkit.
If tiles need repainted they are marked with as dirty and the
thread painting and uploading the textures will only operate on
dirty tiles. This change resulted in some significant refactoring
most of which revolved around these changes...

(1) Removed PaintingInfo from the Texture object and instead track
the state of the tile in Tile object.
(2) Removed all pending TileSets for a TiledPage when the page
produces a new set. This ensures that the tiles currently visible
to the user are painted instead of ones that may already be offscreen.

Change-Id: I93845d8e6e7b066e6bab84bcde11be4a6940002f