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This modifies the eviction order to consider occlusion so that the
LayerEvictionTileIterator and TileManager::EvictionTileIterator will
evict occluded tiles before unoccluded tiles in the same priority bin.

The current eviction order is to evict tiles that are:
1. In a lower priority bin
2. Not required for activation
3. At worse resolution
4. Farther away from visible

This patch adds occlusion to the order to evict tiles that are:
1. In a lower priority bin
2. Not required for activation
3. At worse resolution
4. Occluded
5. Farther away from visible

This patch depends on https://codereview.chromium.org/374653003/ which
tracks occlusion per tree and stores the info on the Tile.

BUG=178971

Review URL: https://codereview.chromium.org/377793003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@283842 0039d316-1c4b-4281-b951-d872f2087c98