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TabActivityTestBase simulates tab navigation and tab switching in a few
unit tests. Split it into its own file, since it's not directly related
to UKM. Also make it a standalone class instead of a test base class.

It was originally a test itself, then spun out into a base class. Now
it uses nothing from ChromeRenderViewHostTestHarness except profile(),
which can be gotten from the TabStripModel. This makes the class's
purpose clearer and allows the UKM helper in tab_ukm_test_helper.h to
be used in browser_tests without including extra test classes.

The remaining class, UkmEntryChecker, should probably be moved
into a renamed file somewhere else in the future, depending on how
useful it is for other teams.

Bug: 784639
Change-Id: I62ca7bb1feaa00e06a39241741696d1f217ed26b
Reviewed-on: https://chromium-review.googlesource.com/903582
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#535972}