Created the GuestView object to manage a guestview from javascript. It exposes a public API to allow the guestview to be created, attached, and destroyed, and hides other implementation details in an internal GuestViewImpl object. It also enforces proper usage/ordering of the API using an internal queue and state machine.

      BUG=431002,434226

Review URL: https://codereview.chromium.org/723453006

Cr-Commit-Position: refs/heads/master@{#304875}