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Removes ThreadHeapMode enum and makes per thread heap mandatory.

Remove HeapTests.{GCParkingTimeout,RecursiveMutex,ObjectDeadBit} because they test threads attached to the main thread's heap.

BUG=591606

Review-Url: https://codereview.chromium.org/2625293002
Cr-Commit-Position: refs/heads/master@{#443190}