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I want to reuse ScopedMockLog from unittests under src/remoting/host.
To do that, I am moving this class from net/test to base/test.

When doing the move, I also wanted to ensure thread-safety in the case when
logging is happening on a thread different from where StartCapturingLogs or
StopCapturingLogs are called.  Having proper locks (and memory barriers implied
by the locks) should ensure that 1) LogMessageHandler won't see a half-way
executed StartCapturingLogs or StopCapturingLogs and 2) that a log write
in-progress won't get a rug pulled from underneath by destroying of gMock's
structures embedded in ScopedMockLog's Log mock method.

BUG=
TEST=net_unittests

Review URL: https://codereview.chromium.org/966423003

Cr-Commit-Position: refs/heads/master@{#319105}