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Currently the instantiation of ChildTraceMessageFilter in
child_thread_impl.cc is guarded by a check on
CommandLine.HasSwitch(kSingleProcess)
The rationale is that in single-process mode we don't want to have any
message filter as the browser-side tracing will cover all the threads.
However, the aforementioned check is not complete, as it does not
covers the case of tests using the InProcessUtilityThreadHelper [1].
In those tests, the kSingleProcess flag is not present, but the
renderer is in-process anyways.
Replacing the the HasSwitch check in with IsInBrowserProcess(),
which covers also the latter case.

[1] for instance extensions/browser/sandboxed_unpacker_unittest.cc

BUG=474973

Review URL: https://codereview.chromium.org/1069253003

Cr-Commit-Position: refs/heads/master@{#324276}