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It specifies the name of the flag-specific expectation file
under web_tests/FlagExpectations and the baseline directory under
web_tests/flag-specific, in the following format:

  {
    "name": "short-name",
    "args": ["--arg1", "--arg2"]
  }

When at least --additional-driver-flag=--arg1 and
--additional-driver-flag=--arg2 are in run_web_tests.py command line,
or --flag-specific=short-name is in the command line,
we will find web_tests/FlagExpectations/short-name for the additional
expectation file and web_tests/flag-specific/short-name for the
additional baseline directory.

Bug: 1019501
Change-Id: Idf0621abc89efc18cd17f9a9612074aa9d7de298
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1894069
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712013}