As discussed in https://chromium-review.googlesource.com/c/chromium/src/+/1048877, we really need some kind of tests for the touch handlers in HWNDMessageHandler because they're fragile and poorly understood.

I investigated doing this with InjectTouchInput and I think it seems feasible. This bug is a reminder to me to follow-up and try to complete this.

Work in progress CL: https://chromium-review.googlesource.com/c/chromium/src/+/1067735

There are two big problems right now: the first is that Chrome doesn't receive the non-client touch events. I believe this is because the test widget is creating a normal Windows window and not a real tabbed browser window. The other is that I think there's a delay before the window starts receiving input, so the test should somehow wait till it's ready (hopefully not via sleeping).