Portals: Keep track of last down motion events

It is possible to activate on touchstart events, and we want to be able
to maintain touch state across activation in that scenario as well.

Bug: 914376
Change-Id: I2970f6184b01f6f9d3f5a4fd81fd3c52b17089bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1661800
Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Adithya Srinivasan <adithyas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671124}
3 files changed