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SerialExtensionHostQueue is now the only implementation of
ExtensionHostQueue. Combine them into a de-virtualized
ExtensionHostQueue.

TBR=halliwell@chromium.org (trivial //chromecast change)

Bug: 1028334
Change-Id: I214596d57ccc1edbd6ba4e9ece266b7845baa954
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1934265
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718919}