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There's some churn going on with extension bindings (to native bindings)
and user gestures (to UAv2). Add two end-to-end bindings tests to expand
our coverage:
- Test that extension messages "pass" the user gesture to the receiver;
  that is, if a user gesture is active when a message is created, it
  should be active on the receiving end as well.
- Test that extension API callbacks are invoked with an active user
  gesture iff the API is invoked when a gesture is active.

Bug: None

Change-Id: I7d4036356bd20fd00c0774210b2019287413e30c
Reviewed-on: https://chromium-review.googlesource.com/1217166Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590162}