chromeos: reenables test in content_browsertest for mash

Specifically MouseLatencyBrowserTest.CoalescedMouseWheelsCorrectlyTerminated,
which is now passing.

PointerEvent was removed a while back, so the DCHECK is no longer hit. I'm not
sure why it was being hit before...

BUG=884342
TEST=none

Change-Id: Ic0089e3b4acac600db0f7723dfdc93d54895b417
Reviewed-on: https://chromium-review.googlesource.com/c/1297572Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602169}

testing/buildbot/filters/chromeos.single_process_mash.content_browsertests.filter

@@ -15,10 +15,6 @@
 -CompositedScrollingBrowserTest.Scroll3DTransformedScroller
 
 
-# DCHECK(!event->IsPointerEvent()) fail. https://crbug.com/884342
--MouseLatencyBrowserTest.CoalescedMouseWheelsCorrectlyTerminated
-
-
 # InputEventAckWaiter::Wait() times out,
 # or RenderWidgetHostMouseEventMonitor::EventWasReceived fails.
 # https://crbug.com/884360