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This patch changes the hash function for the tile map key to use
16 least significant bits from each index. This makes it faster on
systems where sizeof(size_t) == 4.

The collisions would only start happening after a large enough index.

This makes the TilingSetRasterQueueConstructAndIterate test do
about 10% more iterations per second on N4.

BUG=488636
R=enne, danakj

Review URL: https://codereview.chromium.org/1133243010

Cr-Commit-Position: refs/heads/master@{#330448}