Bug 1495964 - Remove unnecessary NS_ASSERTION in nsChildView::DispatchEvent() r=m_kato

The assertion was reasonable check before fixing bug 1446401.  However,
this is currently valid situation in UI Events spec.

Differential Revision: https://phabricator.services.mozilla.com/D7578

--HG--
extra : moz-landing-system : lando

widget/cocoa/nsChildView.mm

@@ -1471,10 +1471,6 @@ nsChildView::DispatchEvent(WidgetGUIEvent* event, nsEventStatus& aStatus)
   debug_DumpEvent(stdout, event->mWidget, event, "something", 0);
 #endif
 
-  NS_ASSERTION(!(mTextInputHandler && mTextInputHandler->IsIMEComposing() &&
-                 event->HasKeyEventMessage()),
-    "Any key events should not be fired during IME composing");
-
   if (event->mFlags.mIsSynthesizedForTests) {
     WidgetKeyboardEvent* keyEvent = event->AsKeyboardEvent();
     if (keyEvent) {