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This CL splits font-display tests into smaller individual tests. The
split has following benefits:

- Clear obstable for implementation of violation report deduplication
- Make the tests stricter. Before the test can verify there are
4 violation reports generated, but cannot tell which 4 cases generate
violation reports.

Previously there was an attempt on isolating tests into subframes
(https://chromium-review.googlesource.com/c/chromium/src/+/2227863),
but failed due to flaky crash in network cache (crbug.com/1087277).

Bug: 1087277
Change-Id: Ifad35b40d112956fb00fd680b329e539a8762ed9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2405916
Commit-Queue: Charlie Hu <chenleihu@google.com>
Reviewed-by: Jason Chase <chasej@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807589}