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This CL adds the ability to pre-warm a compositor process for use by
paint preview show-on-startup.

1. The compositor is started when native is initialized if the
   experiment is enabled.
2. Depending on the situation the compositor is then used or killed.
   a. Compositor is used: disconnect handler is updated and it is used
      as if there was a compositor already.
   b. Compositor is not used: the process is killed immediately.

This moves initialization of the compositor service up to O(100 ms)
earlier (in local tests).

Bug: 1126180
Change-Id: I1e6f627403e8c020b6551c510ad908bba0fc24a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2398562Reviewed-by: Mehran Mahmoudi <mahmoudi@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805317}