Mac: Add rails for scrolling.
Add a MouseWheelRailsFilter structure to compute the rail mode for
input events, based on some simple hysteresis.
Make InputHandlerProxy honor the rails requests of MouseWheel events.
This is not an ideal long-term implementation -- the rails should be
passed to cc::InputHandler, so that they can be handled correctly by
other event types. This version is being used so that it may be merged.
BUG=468454
Review URL: https://codereview.chromium.org/1019153002
Cr-Commit-Position: refs/heads/master@{#321738}
8 files changed
