GitLab

* Modify WebComponent and WebContentRunner to take optional extra
  headers to pass to LoadUrl().

Bug: 977753
Change-Id: I7d23b57629831b775e23ca19a0d0d96886155e6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1672528
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672655}