Refactor GuestViewContainer to split out WebView related logic and MimeHandlerView related logic.

This refactors GuestViewContainer to separate WebView related logic and MimeHandlerView
related logic. Two subclasses are created to contain the logic related to each. The
content API has to be modified to allow creating each of the two types of container.

BUG=303491

Review URL: https://codereview.chromium.org/702433002

Cr-Commit-Position: refs/heads/master@{#303118}
13 files changed