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This was caused by some very slow running trybots. If the test was running significantly slower
than a second, a MemoryPressureNotifier event was able to be sent, which caused the ResourceManager
to perform some asynchronous action which is not wanted for these unit tests.

BUG=412537, 417189
TEST=AppActivityTes.TestUnloadProxyLocation, AppActivityTest.TestMultipleActivityUnloadLock

Review URL: https://codereview.chromium.org/598033004

Cr-Commit-Position: refs/heads/master@{#296545}