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Portals can be activated in response to scroll events like overscroll
and we want users to be able to continue scrolling after activation
without tapping the screen again. This CL dispatches touch events
targeted at the new ContentView after activation, which mirror where
the pointers were before activation.

Design doc: https://docs.google.com/document/d/12EsIEQIJAzh4-3aNQNhxoQ8Z7ayrSodmWbo06R9PbAQ/edit?usp=sharing

Bug: 914376
Change-Id: I1ade7ac3da7451de14b38dd66056178cdcab7631
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1622906
Commit-Queue: Adithya Srinivasan <adithyas@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665179}