Support pen inputs in web platform tests

Because pen inputs are very similar to mouse inputs, for now, we use
mouse events to represent pen events with pointer type of "pen".

Bug: chromedriver:1897
Change-Id: I0c830f65a1f1bdf1bc6d4a5ef042fc333d18d4ca
Reviewed-on: https://chromium-review.googlesource.com/c/1381144
Reviewed-by: Joel Einbinder <einbinder@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Lan Wei <lanwei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619474}
10 files changed