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This works around the fact that the 10.14 config is not complete and
doesn't work for [ Mac ] expectations.

Also refactors our test_suites.pyl entries to deduplicate our giant
list of command line flags and tighten down on unnecessary flags on
some systems.

Bug: 984818   # WebGL CTS tracking issue
Bug: 1014785  # Worked-around expectations bug
Change-Id: I215a01a349262cecdf54bcbc6f6c8137b4426f5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1867990Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707539}