Adds source to Assistant URLs.

Assistant Reminders, Settings, etc. needs to be able to distinguish
between being loaded in the browser and being embedded in Assistant UI.
They need this information to fork behavior and UI treatments, such as
deep links that should only be provided if embedded UI or top bars that
should only be shown if in the browser.

To indicate that we are in an embedded state, we will use a URL
parameter: |source|.

Note that we will default |source| to Assistant, and can support
parameterizing this value later if need be.

Bug: b:128863104
Change-Id: Ia76a53a5c68352313a0015154f5793fad0e8c9cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529918
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643058}
2 files changed