GitLab

The PointerEvent spec can't gracefully support a hovering pen in eraser
mode because a non-zero |buttons| field indicate an active buttons
state. The issue is currently being discussed in github:
  https://github.com/w3c/pointerevents/issues/134

Until the spec gets fixed, this CL exposes the eraser mode in a
spec-compliant way by hiding the eraser mode until the pen touches the
digitizer. Edge shows the same behavior.

This CL also adds X1, X2 & Eraser button/buttons values to
WebPointerProperties.

BUG=642455

Review-Url: https://codereview.chromium.org/2296303002
Cr-Commit-Position: refs/heads/master@{#416360}