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Transient XRInputSources need to be added and removed from the input
source list as they are needed.  This exposes two helper methods on
XRSession to add/remove a transient source, and updates the canvas input
provider to call those methods when sending events.

Bug: 968229
Change-Id: I49e0313217a9d1526722c8a9f1e57f1141d3ee3f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1635775Reviewed-by: Bill Orr <billorr@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664850}