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Many of these tests previously worked by using a pattern such as:
. make TabScrubbers timer use a zero delay.
. generate a bunch of async scroll events (using EventGenerator)
. record when active tab changes.
. run message loop, waiting for active tab to change to certain index,
  then stop.

This is problematic in the mash case as EventGenerator, for mash, runs
a message loop. This meant the timer would fire at unexpected times.

The fix is to increase the timer delay, generate a single event and then
force the TabScrubber timer to complete. This way the test gets consistent
results, regardless of whether EventGenerator internally runs a message
loop.

BUG=889097,890050
TEST=test only changes

Change-Id: I5decb690c4ef7ff67af21cc9a588aaa5046fad89
Reviewed-on: https://chromium-review.googlesource.com/c/1297288Reviewed-by: Jun Mukai <mukai@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602522}