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Increase shard count used for the debug and msan/asan builders to prevent
the tests from timing out.  Repurposes shards previously used for
mash_browser_tests, as crrev.com/c/1175222 reduced the number of tests running
as part of that suite to just 1.

Also sets the timeout for tests back to 1800.  It was bumped up to 3600
by crrev.com/c/1217124.

Bug: 874090
Change-Id: Ia2b0ea4d3264e240dadde04f8ff049ff9a8b5735
Reviewed-on: https://chromium-review.googlesource.com/1217124Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Ryan Cui <rcui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590437}