Revert of cc: Remove tile sharing from tilings.

Revert of cc: Remove tile sharing from tilings. (patchset #20 id:380001 of https://codereview.chromium.org/1051993002/)

Reason for revert:
Several failures happened as a result.

Original issue's description:
> cc: Remove tile sharing from tilings.
>
> This patch removes sharing tiles from tilings. Previously, tiles
> that were not invalidated were shared between the pending and
> the active trees. With this patch, tiles that are exist on the
> active tree and are not invalidated are not created on the pending
> tree, resulting in fewer overall tiles. This improves performance
> of updating and managing tiles.
>
> Also, this patch opens up opportunities for a lot of code clean
> up that can otherwise be confusing when two trees are involved.
>
> R=danakj, enne
>
> Committed: https://crrev.com/0061b75e810f088900a0a8fbdd29ba5595c73016
> Cr-Commit-Position: refs/heads/master@{#326804}

TBR=enne@chromium.org,danakj@chromium.org,timford35@gmail.com
BUG=481421, 481509, 481480
Committed: https://crrev.com/3bc40f2782615107e6a8f41b056a6477275f75a8
Cr-Commit-Position: refs/heads/master@{#327093}

[vmpstr, 2015-04-27 17:03:35]

Created Revert of cc: Remove tile sharing from tilings.

[vmpstr, 2015-04-27 17:04:51]

The CQ bit was checked by vmpstr@chromium.org

[commit-bot: I haz the power, 2015-04-27 17:05:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1108773003/1

[commit-bot: I haz the power, 2015-04-27 19:04:45]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-27 19:06:30]

Patchset 1 (id:??) landed as
https://crrev.com/3bc40f2782615107e6a8f41b056a6477275f75a8
Cr-Commit-Position: refs/heads/master@{#327093}