Support calling webrtcLoggingPrivate from an app for a webview.

Adds a flag to webrtcLoggingPrivate's RequestInfo that causes the API
functions to look up the render process id for a webview in the caller's
web contents.

There are 2 ways webrtcLoggingPrivate functions can be called.

  1. From a whitelisted component extension on behalf of a page with the
  appropriate origin via a message from that page. In this case, either the
  guest process id or the tab id is on the message received by the component
  extension, and the extension can pass that along in RequestInfo as
  |guest_process_id| or |tab_id|.

  2. From a whitelisted app that hosts a page in a webview. In this case,
  the app should call these API functions with the |target_webview| flag
  set, from a web contents that has exactly 1 webview .

Case 1 was already supported before. This change adds support for case 2.

Bug: 777933
Change-Id: I02f6d011fc7a6f1589c10aff19dc6e6e8f2e24a8
Reviewed-on: https://chromium-review.googlesource.com/763911
Commit-Queue: Adrian Elder <[email protected]>
Reviewed-by: Devlin <[email protected]>
Cr-Commit-Position: refs/heads/master@{#517859}
7 files changed