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Once a sample is being recorded we close the "fast path" (lock free operation)
of RecordFree function. That was bacause insertion of an element into
unordered_map may cause rehashing, making all concurrent read operations
invalid.

The patch makes it be less strict, by locking the fast path only when there's a
chance for the container do rehashing.

Change-Id: I80553f59400f97ada7a2a6097b201c2da62d3b2e
Reviewed-on: https://chromium-review.googlesource.com/1056404
Commit-Queue: Alexei Filippov <alph@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558439}