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CastWebContentsImpl now has an associated browser test. This allows us
to test various content scenarios, such as net errors, content errors,
and page crashes. It also tests the standard lifecycle of a
CastWebContents, and verifies various state transitions. This new test
prompted some modifications to the CastWebContentsImpl class, including
some more descriptive comments on CWC's intended behavior.

Bug: internal b/67864604
Bug: internal b/130190264
Test: cast_shell_browsertests

Change-Id: Ibe24207cf091634cb046898276258579d9234b9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1560492
Commit-Queue: Sean Topping <seantopping@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649697}