Portals: Implement postMessage for HTMLPortalElement

Implements a version of postMessage for HTMLPortalElement that can only
send strings. The message is sent to the PortalHost object in the
portal's main frame. Support for a more general message type will be
added in a follow-up CL.

Bug: 914120
Change-Id: I6366247f6c699eb8c41458e676b96bd81c70e5a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1423462
Commit-Queue: Adithya Srinivasan <adithyas@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Lucas Gadani <lfg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643369}
27 files changed