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This patch ensures that we heapify pointers instead of objects in
priority queue eviction.

Performance data:
Before:
tile_manager_eviction_tile_queue_construct:
2= 411875.46875 runs/s
10= 8762.1435546875 runs/s
50= 48.008880615234375 runs/s

tile_manager_eviction_tile_queue_construct_and_iterate:
2_16= 3337.1767578125 runs/s
2_32= 3041.9306640625 runs/s
2_64= 2513.700439453125 runs/s
2_128= 1940.756591796875 runs/s

After:
tile_manager_eviction_tile_queue_construct:
2= 391767.84375 runs/s (-5%)
10= 8705.869140625 runs/s (-2%)
50= 49.93892288208008 runs/s (+4%)

tile_manager_eviction_tile_queue_construct_and_iterate:
2_16= 3691.25146484375 runs/s (+10%)
2_32= 3666.619384765625 runs/s (+20%)
2_64= 3544.351318359375 runs/s (+41%)
2_128= 3341.751708984375 runs/s (+72%)

R=reveman

Review URL: https://codereview.chromium.org/430053003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287846 0039d316-1c4b-4281-b951-d872f2087c98