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This CL allows CastWebContentsActivity to be re-launched multiple times
for a single session, while re-attaching the existing WebContents.

Bug: internal b/151099227
Test: back out of cast app and resume with NowPlaying or WatchNext

Change-Id: I280cf45f1153ad9238e8006c01349c1c6798ae0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2161729Reviewed-by: Simeon Anfinrud <sanfin@chromium.org>
Commit-Queue: Thoren Paulson <thoren@chromium.org>
Cr-Commit-Position: refs/heads/master@{#763595}