RC: Do not initialize TabManager in ChromeUnitTestSuite.

This CL removes the initialization of TabManager by ChromeUnitTestSuite
before every Chrome unit test run. This will make it easier to
initialize metrics::DesktopSessionDurationTracker before TabManager in
tests that need to verify that Chrome usage (instead of wall time) is
used for tab lifecycle state transitions.

Bug: 775644
Change-Id: I01d05579964822c87ebc515cafdd25e08fdab661
Reviewed-on: https://chromium-review.googlesource.com/1132121
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574165}
4 files changed