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This allows custom implementations that can share base functionality.
This is needed since instances must be created to control both web views
and cast apps.

Bug: b/136460199
Test: Compile cast_shell
Change-Id: Id19507b3127f4bd2071e02f4b2cd4ec931c7ea71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872466Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Shawn Gallea <sagallea@google.com>
Cr-Commit-Position: refs/heads/master@{#707915}