GitLab

Adds new experimental protocol method, which uses
`WebContents::GenerateMHTML` underneath.

Bug: 901933
Change-Id: I52e707f89378fe3a1488a3cd8afd7cdded1b77ea
Reviewed-on: https://chromium-review.googlesource.com/c/1297732
Commit-Queue: Erik Luo <luoe@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606256}