[ChromeDriver] Set key modifiers with mouse actions

When we have keyboard events and mouse events send together, we should
keep the keys that we are pressing now and set them to the mouse events
that are in the same action sequence.

Bug: chromedriver:1897
Change-Id: I700d7a87d3d0a1b45c1588318ab21314aa284417
Reviewed-on: https://chromium-review.googlesource.com/c/1482009
Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Lan Wei <lanwei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634700}
4 files changed