[content-service] Support basic View embedding

Adds support for embedding a View object within a client application's
window tree. Currently Chrome OS only.

Enables a new simple_browser test application to be launched via a
command-line flag in DCHECK-enabled builds only, for manual developer
testing. This application runs in a sandboxed utility process and embeds
a View that can be navigated, displaying web contents within the
application's UI, backed by a concrete WebContents owned by a Content
Service instance in the browser process.

Bug: 855092
Change-Id: I95937f987e87ad303689d73b7e57c836ee706d28
Test: Launch Chrome OS chrome --enable-features=Mash --launch-simple-browser and verify that you can navigate the cute little browser window that pops up.
Reviewed-on: https://chromium-review.googlesource.com/1125342
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574568}
38 files changed