cc: Add occlusion checker as a fixed view of occlusion tracker.

This patch adds an occlusion checker to get the state of
occlusion tracker for permanent checks. That is, occlusion
checker does not have to be queried during a layer walk.

BUG=410932
R=danakj

Review URL: https://codereview.chromium.org/547723002

Cr-Commit-Position: refs/heads/master@{#294205}

• cc/BUILD.gn[diff]
• cc/cc.gyp[diff]
• cc/layers/layer_impl.h[diff]
• cc/layers/picture_image_layer_impl_unittest.cc[diff]
• cc/layers/picture_layer_impl.cc[diff]
• cc/layers/picture_layer_impl.h[diff]
• cc/layers/picture_layer_impl_unittest.cc[diff]
• cc/resources/picture_layer_tiling.cc[diff]
• cc/resources/picture_layer_tiling.h[diff]
• cc/resources/picture_layer_tiling_perftest.cc[diff]
• cc/resources/picture_layer_tiling_unittest.cc[diff]
• cc/resources/tile_manager_perftest.cc[diff]
• cc/resources/tile_manager_unittest.cc[diff]
• cc/trees/layer_tree_impl.cc[diff]
• cc/trees/occlusion.cc[Added - diff]
• cc/trees/occlusion.h[Added - diff]
• cc/trees/occlusion_tracker.cc[diff]
• cc/trees/occlusion_tracker.h[diff]
• cc/trees/occlusion_tracker_unittest.cc[diff]