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This uses the generic allocator shim to hook into heap allocations.
When disabled and unused, there is no runtime penalty for this. When heap tracing is enabled, there's a small accounting overhead for every allocation.

Instantiating a ScopedThreadHeapUsage instance carries O(1) cost, whether or not heap tracing is enabled.

BUG=644385

Review-Url: https://codereview.chromium.org/2163783003
Cr-Commit-Position: refs/heads/master@{#417601}