[Chromecast] Fix the flashing page when destroy web content

There is a flashing page when transiting from one cast app to
another cast app or when stopping a cast app. It is caused by redrawing
the webcontent view when destroying the WebContents.

Bug: internal b/79172781 b/79694573

Test: manually tested on Jasper

Change-Id: Ib445301a1f0d2a79cdf5a7dd2652a0666c570f06
Reviewed-on: https://chromium-review.googlesource.com/1132622
Reviewed-by: Simeon Anfinrud <sanfin@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Zhiheng(Vincent) Li <vincentli@google.com>
Cr-Commit-Position: refs/heads/master@{#574794}