Add support for native event processor observer to headless mode.

The hooks are currently required by the jank calculator. They will eventually be
the mechanism that the browser task scheduler hooks into native events.

Bug: 859155
Change-Id: I4b633c7b350c673c7b59cdcfe3a115ae74e5bacb
Reviewed-on: https://chromium-review.googlesource.com/1238777Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593575}

headless/lib/browser/headless_shell_application_mac.mm

@@ -5,6 +5,15 @@
 #include "headless/lib/browser/headless_shell_application_mac.h"
 
 #include "base/auto_reset.h"
+#include "base/observer_list.h"
+#include "content/public/browser/native_event_processor_mac.h"
+#include "content/public/browser/native_event_processor_observer_mac.h"
+
+@interface HeadlessShellCrApplication ()<NativeEventProcessor> {
+  base::ObserverList<content::NativeEventProcessorObserver>::Unchecked
+      observers_;
+}
+@end
 
 @implementation HeadlessShellCrApplication
 
@@ -17,4 +26,20 @@
 - (void)setHandlingSendEvent:(BOOL)handlingSendEvent {
 }
 
+- (void)sendEvent:(NSEvent*)event {
+  content::ScopedNotifyNativeEventProcessorObserver scopedObserverNotifier(
+      &observers_, event);
+  [super sendEvent:event];
+}
+
+- (void)addNativeEventProcessorObserver:
+    (content::NativeEventProcessorObserver*)observer {
+  observers_.AddObserver(observer);
+}
+
+- (void)removeNativeEventProcessorObserver:
+    (content::NativeEventProcessorObserver*)observer {
+  observers_.RemoveObserver(observer);
+}
+
 @end