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- TAP_DOWN is added to CastContentWindow::GestureType
- The gesture detection is attached to the root window and handles
  tap down events. A new JS binding API: onTapDownHandler will be hosted on
  platform
- HandleTapGesture: finger is released after pressed within tap timeout window
  HandleTapDownGesture: finger is pressed

Bug: internal b/75979846
Test: build manually and test whether both tap and tap down handler callback works
Change-Id: Iaefa6df9c7a5284ba27964083fc056bf36cf480a
Reviewed-on: https://chromium-review.googlesource.com/1115418Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Jiawei Li <lijiawei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574373}