GitLab

This implements:

- allows the content embedder to merge a manifest into the base ones provided by content, allowing it to add to the list of interfaces exposed to the renderer.
- adds such an overlay to chrome/browser

So.. from now on if you add an interface to either the browser/renderer, you'll need to list it in the manifest. I wonder how we should broadcast this. Also as a separate step we should require these manifests get security review.

R=rockot@chromium.org,tsepez@chromium.org

Committed: https://crrev.com/4ea17059bf78553528f3ffb4a9de84a447622fd3
Committed: https://crrev.com/807a926c9ca02ac8ae1b52e82537834ef78fb9ba
Committed: https://crrev.com/91c25a3cedc4fadcb8c84e91cc59a8f90411058a
Review-Url: https://codereview.chromium.org/2259903002
Cr-Original-Original-Original-Commit-Position: refs/heads/master@{#418282}
Cr-Original-Original-Commit-Position: refs/heads/master@{#418398}
Cr-Original-Commit-Position: refs/heads/master@{#418582}
Cr-Commit-Position: refs/heads/master@{#418691}