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On MacOS the implementation of libmalloc sometimes calls malloc recursively,
delegating allocations between zones. That causes our hooks being called
twice. The scoped guard allows us to detect that, and avoid double counting.

BUG=803276

Change-Id: I96bfa2c7cf29b0f21777033f259c2e2d8286b2e2
Reviewed-on: https://chromium-review.googlesource.com/1187831
Commit-Queue: Alexei Filippov <alph@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585711}