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In crbug.com/1049126, we discovered that origin trials are not enabled
when running in headless mode. This was not intentional, as headless
should behave the same as regular Chrome with respect to origin trials.

The root cause is that origin trials require the embedder (i.e. Chrome)
to explicitly opt-in to enable trials. The opt-in is done by providing
an implementation of OriginTrialPolicy [1]. It turns out that headless
is actually a separate embedder than Chrome, which was missed in the
initial implementation.

A workaround for the problem has been landed in [2], and merged back to
M80. This CL is intended to be merged back to M81.

This CL implements the opt-in for the headless embedder, by copying the
Chrome embedder implementation. Ideally, headless should re-use the
Chrome implementation, but that would require larger changes. The
duplication will be addressed in M82 in crbug.com/1049317.

[1] https://source.chromium.org/chromium/chromium/src/+/master:third_party/blink/public/common/origin_trials/origin_trial_policy.h
[2] https://chromium-review.googlesource.com/c/chromium/src/+/2039601

Bug: 1049126
Change-Id: Ide596c100baae4b69eab2149169eb92353a7bac7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2042382
Commit-Queue: Jason Chase <chasej@chromium.org>
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Johannes Henkel <johannes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739253}