Makes touch event have a time from android

Also documents what Event::time_stamp is.

R=erg@chromium.org

Review URL: https://codereview.chromium.org/1042363003
diff --git a/mojo/services/input_events/public/interfaces/input_events.mojom b/mojo/services/input_events/public/interfaces/input_events.mojom
index 0086265..9fa3630 100644
--- a/mojo/services/input_events/public/interfaces/input_events.mojom
+++ b/mojo/services/input_events/public/interfaces/input_events.mojom
@@ -70,6 +70,8 @@
   EventType action;
   // TODO(sky): parts of this should move to PointerData.
   EventFlags flags;
+  // Time the event was delivered. The time is in milliseconds and corresponds
+  // to the uptime of the machine.
   int64 time_stamp;
   KeyData? key_data;
   PointerData? pointer_data;
diff --git a/services/native_viewport/platform_viewport_android.cc b/services/native_viewport/platform_viewport_android.cc
index 2770073..104c183 100644
--- a/services/native_viewport/platform_viewport_android.cc
+++ b/services/native_viewport/platform_viewport_android.cc
@@ -114,6 +114,7 @@
                                          jfloat h_wheel,
                                          jfloat v_wheel) {
   mojo::EventPtr event(mojo::Event::New());
+  event->time_stamp = time_ms;
   event->action = MotionEventActionToEventType(masked_action);
   if (event->action == mojo::EVENT_TYPE_UNKNOWN)
     return false;