Decouple EventWithLatencyInfo and WebInputEventTraits [extern templates]

content/common/input/event_with_latency_info.h

Index: content/common/input/event_with_latency_info.h
diff --git a/content/common/input/event_with_latency_info.h b/content/common/input/event_with_latency_info.h
index 24742a3baeb0f57f6755724c5cd33ba93272c6b3..c224201c2fb22ed26aa8d064ea6fe765efb9bda9 100644
--- a/content/common/input/event_with_latency_info.h
+++ b/content/common/input/event_with_latency_info.h
@@ -5,17 +5,12 @@
 #ifndef CONTENT_COMMON_INPUT_EVENT_WITH_LATENCY_INFO_H_
 #define CONTENT_COMMON_INPUT_EVENT_WITH_LATENCY_INFO_H_
 
+#include "base/compiler_specific.h"
+#include "content/common/content_export.h"
+#include "ipc/export_template.h"
+#include "third_party/WebKit/public/web/WebInputEvent.h"
 #include "ui/events/latency_info.h"
 
-#include "content/common/input/web_input_event_traits.h"
-
-namespace blink {
-class WebGestureEvent;
-class WebMouseEvent;
-class WebMouseWheelEvent;
-class WebTouchEvent;
-}
-
 namespace content {
 
 template <typename T>
@@ -24,31 +19,40 @@ class EventWithLatencyInfo {
   T event;
   mutable ui::LatencyInfo latency;
 
+  EventWithLatencyInfo() {}
   explicit EventWithLatencyInfo(const T& e) : event(e) {}
-
   EventWithLatencyInfo(const T& e, const ui::LatencyInfo& l)
       : event(e), latency(l) {}
 
-  EventWithLatencyInfo() {}
+#if defined(OS_WIN)
+  // Explicitly declare these, otherwise invoking the move constructor via
+  // dllimport on Windows generates a link error. See http://crbug.com/623844.
+  // Declare all three otherwise the move constructor will hide the others.
+  EventWithLatencyInfo(const EventWithLatencyInfo& other) = default;
+  EventWithLatencyInfo(EventWithLatencyInfo&& other) = default;
+  EventWithLatencyInfo& operator=(const EventWithLatencyInfo& other) = default;
+#endif
 
-  bool CanCoalesceWith(const EventWithLatencyInfo& other)
-      const WARN_UNUSED_RESULT {
-    return WebInputEventTraits::CanCoalesce(other.event, event);
-  }
-
-  void CoalesceWith(const EventWithLatencyInfo& other) {
-    // |other| should be a newer event than |this|.
-    if (other.latency.trace_id() >= 0 && latency.trace_id() >= 0)
-      DCHECK_GT(other.latency.trace_id(), latency.trace_id());
-    WebInputEventTraits::Coalesce(other.event, &event);
-    // When coalescing two input events, we keep the oldest LatencyInfo
-    // for Telemetry latency tests, since it will represent the longest
-    // latency.
-    other.latency = latency;
-    other.latency.set_coalesced();
-  }
+  bool CanCoalesceWith(const EventWithLatencyInfo& other) const
+      WARN_UNUSED_RESULT;
+  void CoalesceWith(const EventWithLatencyInfo& other);
 };
 
+// The required types are known beforehand, so avoid compiling the required
+// template in each translation unit that instantiates one by declaring them
+// extern. This also allows some internals to be deferred to the .cc. If another
+// type is instantiated, the compiler will generate code for everything except
+// [Can]CoalesceWith(): If those methods are also _invoked_ there will be a link
+// error unless they're defined somewhere as well.
+extern template class EXPORT_TEMPLATE_DECLARE(CONTENT_EXPORT)
+    EventWithLatencyInfo<blink::WebGestureEvent>;
+extern template class EXPORT_TEMPLATE_DECLARE(CONTENT_EXPORT)
+    EventWithLatencyInfo<blink::WebMouseWheelEvent>;
+extern template class EXPORT_TEMPLATE_DECLARE(CONTENT_EXPORT)
+    EventWithLatencyInfo<blink::WebMouseEvent>;
+extern template class EXPORT_TEMPLATE_DECLARE(CONTENT_EXPORT)
+    EventWithLatencyInfo<blink::WebTouchEvent>;
+
 typedef EventWithLatencyInfo<blink::WebGestureEvent>
     GestureEventWithLatencyInfo;
 typedef EventWithLatencyInfo<blink::WebMouseWheelEvent>