Fix possibly flakiness of EtwTraceControllerTest.EnableDisable
Fix possibly flakiness of EtwTraceControllerTest.EnableDisable

The last provider.WaitForCallback() returns immediately because the
callback event of the previous controller.EnableProvider() has not
been consumed. If controller.Stop() is asynchronous, the last two
EXPECT_EQs will fail.

Add a provider.WaitForCallback() call before controller.Stop() to
consume the previous callback event.

Found this because https://codereview.chromium.org/1345223004/
mysteriously caused failure of the test.

BUG=525297

Review URL: https://codereview.chromium.org/1353893003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#349523}

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