Enable SignedHTTPExchange feature by default

Intent-to-Ship thread: https://groups.google.com/a/chromium.org/d/msg/blink-dev/gPH_BcOBEtc/Z41GR0mwEQAJ

Bug: 803774, 877358
Change-Id: I494d1a8156f0d0ba9b1f010aeb3c758d7c6add02
Reviewed-on: https://chromium-review.googlesource.com/c/1451498
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Auto-Submit: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628657}
7 files changed