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Now that the service manager has moved back into the browser process
these tests have become reliable again. The last 100 FYI runs have had
other test suites fail, but not mash_browser_tests:

https://ci.chromium.org/buildbot/chromium.fyi/Mojo%20ChromiumOS/?limit=100

* Eliminate single_mash_browser_tests step (which was running 1 test) and
  fold into mash_browser_tests
* Put mash_browser_tests back with ~125 tests
* Synchronize mash.browser_tests.filter with mojo.fyi.browser_tests.filter

Bug: none
Test: bots
Change-Id: I44d5280ab69dcd1c27c11228737b357bece22154
Reviewed-on: https://chromium-review.googlesource.com/722107Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509644}