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This was implemented in crbug.com/633007 but was never adopted
by any standards, and could not get much attention from developers
except for testing use cases.

As this was dismissed from the standard, we have to pursue the
testing use cases in other forms.

Bug: 829713
Change-Id: I2b053a15e96d68ff5a8c632e65b7b2feefb15dab
Reviewed-on: https://chromium-review.googlesource.com/999453Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Takayoshi Kochi <kochi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575599}