Add WebContentController

Split WebviewController into a generic base class that only
deals with mutating the WebContents. The split allows implementing
a Webview and CastApp controller differently while retaining common
functionality in the base class.


Bug: b/136460199
Test: Compile
Change-Id: I4267e3c6ac4e3904160e7ee34e35d2b220777cad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1867189
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Shawn Gallea <sagallea@google.com>
Cr-Commit-Position: refs/heads/master@{#707487}