cc: Make the TileManager operate on ManagedTileState directly

cc: Make the TileManager operate on ManagedTileState directly

Change the TileManager to operate on ManagedTileState, so that
it can be made unaware of Tiles that are unimportant (rather than
storing them in separate structures).

Change the TileMap to hold a TileHandle structure, which holds
a scoped_refptr to a separate Tile and ManagedTileState. These
are always allocated together, but will be separated soon.

Make the ManagedTileState hold a scoped_refptr to its owning Tile.
This allows the patch to minimize risk by not trying to duplicate
and track the state of hte Tile in the ManagedTileState. It may
be that future patches will want to do this.

The destruction order is that the PictureLayerTiling will clear
its TileMap, which will cause the TileHandle to be freed,
which will cause the ManagedTilesTate to be freed, which will
cause (or allow) the Tile state to be freed.

BUG=165852

[ccameron, 2013-02-16 13:38:24]

This is patch 1 of 3 patches which make the TileManager only be aware of the
alive tiles.

This is resolved version of reveman's change from
https://codereview.chromium.org/12082086/

The only significant difference between this change and reveman's change is that
the ManagedTileState holds a (always-valid) back-pointer to its owning Tile,
rather than mirroring the state of the Tile. This minimized the diffs and the
fragility of the change, so I don't advocate trying anything different until
M25.

This diff touches a lot of lines, so I explicitly tried make the diff as small
and brain-dead as possible.

[reveman, 2013-02-19 01:40:31]

I like this incremental approach. Thanks for doing this.