[XProto] Fix crash in media::UserInputMonitorLinuxCore::DispatchXEvent

The X server may send unsolicited events (from clients calling
XSendEvent() or from new keyboard mappings).  So we must check that
the event is the correct type before dispatching it.

R=dalecurtis

Change-Id: Ic3e19f7da1415c358991609c9b016bd60ead2038
Bug: 1140927
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490674
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819650}

media/base/user_input_monitor_linux.cc

@@ -115,9 +115,10 @@ void UserInputMonitorLinuxCore::DispatchXEvent(x11::Event* event) {
   DCHECK(io_task_runner_->BelongsToCurrentThread());
 
   auto* raw = event->As<x11::Input::RawDeviceEvent>();
-  DCHECK(raw);
-  DCHECK(raw->opcode == x11::Input::RawDeviceEvent::RawKeyPress ||
-         raw->opcode == x11::Input::RawDeviceEvent::RawKeyRelease);
+  if (!raw || (raw->opcode != x11::Input::RawDeviceEvent::RawKeyPress &&
+               raw->opcode != x11::Input::RawDeviceEvent::RawKeyRelease)) {
+    return;
+  }
 
   ui::EventType type = raw->opcode == x11::Input::RawDeviceEvent::RawKeyPress
                            ? ui::ET_KEY_PRESSED