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This patch added PageLoadMetricsObserverContentTestHarness in component,
so chrome's observer unittests can be easily move to components. The
main changes are
- Moved PageLoadMetricsObserverTester to components
- Moved PageLoadMetricsTestContentBrowserClient to separated files for sharing.
- Moved use_counter_page_load_metrics_observer.cc to components to
  verify PageLoadMetricsObserverContentTestHarness working.
- The other observers changes are renaming.

Bug: 995880
Change-Id: I4f64d8c8941c586d5b44791911ddbc0606b6061c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1838474Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Tao Bai <michaelbai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704690}