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This CL also makes sure that the default base::MemoryPressureMonitor
won't be created when running browser tests.  This helps insulate the
tests from their environment and ensure that the tests behave
deterministically (rather than flakily depending on things outside the
test control, like the overall memory pressure on the bot which runs the
test).  This is accomplished by avoiding creating the monitor if
--browser-test switch is present.

Before this CL, the --browser-test switch would be injected by
ContentBrowserTest::SetUp.  After this CL, the switch is injected by
BrowserTestBase::SetUp - this means that insulation from memory pressure
covers not only content_browsertests, but also browser_tests (and other
tests that subclass BrowserTestBase rather than ContentBrowserTest).

This CL also makes tweaks in memory-pressure-monitoring code to make
sure that things work properly after the changes described above.
In particular, the CL also makes the following changes:

- Move DummyMemoryPressureMonitor from
  //chrome/browser/chromeos/resource_reporter/resource_reporter_unittest.cc
  into //base (renaming the class into FakeMemoryPressureMonitor), so
  that it can be reused by TabManagerTest.OomPressureListener browser
  test.  Note that the FakeMemoryPressureMonitor has to be constructed
  and present early during browser startup (so that TabManager::Start
  detects presence of the monitor).

- Stop incorrect downcasting that used to be made by
  base::chromeos::MemoryPressureMonitor::Get

There are also some opportunistic changes that are not really required
for correctness:

- Rewrite TabManager::OnMemoryPressure to follow most recent
  style guidelines that help enforce that missing a switch/case
  branch will lead to a compile error (thanks to a missing "default"
  branch).

Bug: 852905
Change-Id: I6eb651a4f7a7bc6d1a2138ba31dd248c7e33f2a4
Reviewed-on: https://chromium-review.googlesource.com/1194879
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590331}