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Each WebContents can have an arbitrary number of RenderViewHosts. Since
each RenderViewHost would re-configure their WebPreferences, cast
webviews need to cache the user-configured preferences such that they
can be applied to new RenderViewHosts.

Bug: b/163595220
Test: On device load page with autoplay video.
Change-Id: I546f80a7271204aa2c5ee5939054bc949044d977
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2366474Reviewed-by: Luke Halliwell (slow) <halliwell@chromium.org>
Reviewed-by: Albert Chaulk <achaulk@chromium.org>
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800207}