[THSP] Move the ContentsContainer and SidePanel into container view

Reparent the ContentsContainer and Sidepanel views into a new container
called BrowserMainView.

In order to create a modular system that supports the toolbar height
side panel we are moving all views minus the TopContainer into
BrowserMainView. This is so all of the applicable views can be resized
and animated together.

Followup CLs will address the Bookmarksbar, Infobar, Toolbar and
immersive mode interactions.

Tests have been updated to reflect the new structure.

Change-Id: I18fa40008380d045fdd0f11d954219cf581ec497
Bug: 445446905
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6962035
Commit-Queue: Darryl James <dljames@chromium.org>
Reviewed-by: Caroline Rising <corising@chromium.org>
Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1520173}