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This removes SignedHTTPExchangeOriginTrial feature, and its accompanied
SignedHTTPExchangeAcceptHeader feature which is used for the Accept
header opt-in.

Bug: 827939,887201
Change-Id: I0794b886715a63cb6ac43c37a0adaddeb31f2af8
Reviewed-on: https://chromium-review.googlesource.com/c/1481181Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634519}