[tracing] fix a data race in MemoryDumpManager

[tracing] fix a data race in MemoryDumpManager

Fix a data race spotted by the TSan bot [1] and re-enable the
MemoryDumpManagerTest.RespectTaskRunnerAffinity test.
The race consisted in mistakenly looking into the
pmd_holder->num_pending_async_requests to determine whether the dump
should be finalized inline at the end of the current stack frame
or on the last thread hop.
In theory it is possible that the chain of posted threads is done
before hitting the num_pending_async_requests check.

[1] http://build.chromium.org/p/chromium.memory.fyi/builders/Linux%20TSan%20Tests/builds/5757

BUG=499983
Committed: https://crrev.com/5dc4c114e510b6d6a9d0c5760a8fba17186d0966
Cr-Commit-Position: refs/heads/master@{#334396}

[Primiano Tucci (use gerrit), 2015-06-15 12:07:30]

primiano@chromium.org changed reviewers:
+ petrcermak@chromium.org

[petrcermak, 2015-06-15 14:00:26]

LGTM.

Thanks,
Petr

https://codereview.chromium.org/1186053006/diff/1/base/trace_event/memory_dum...
File base/trace_event/memory_dump_manager.cc (right):

https://codereview.chromium.org/1186053006/diff/1/base/trace_event/memory_dum...
base/trace_event/memory_dump_manager.cc:320: did_post_any_async_task |=
did_post_async_task;
As Simon suggested, this could be "... = true;" in the if block on the previous
line.

[Primiano Tucci (use gerrit), 2015-06-15 14:02:33]

The CQ bit was checked by primiano@chromium.org

[Primiano Tucci (use gerrit), 2015-06-15 14:02:33]

The patchset sent to the CQ was uploaded after l-g-t-m from
petrcermak@chromium.org
Link to the patchset: https://codereview.chromium.org/1186053006/#ps20001
(title: "femto perf improvement")

[commit-bot: I haz the power, 2015-06-15 14:04:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1186053006/20001

[commit-bot: I haz the power, 2015-06-15 17:06:49]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-06-15 17:07:37]

Patchset 2 (id:??) landed as
https://crrev.com/5dc4c114e510b6d6a9d0c5760a8fba17186d0966
Cr-Commit-Position: refs/heads/master@{#334396}