Make keyboard shortcuts work on Mac

When the renderer doesn't handle a key event, we pass it off to the main menu
to see if it can handle it.

Part of #2.

brightray/browser/default_web_contents_delegate.cc

@@ -0,0 +1,11 @@
+#include "browser/default_web_contents_delegate.h"
+
+namespace brightray {
+
+DefaultWebContentsDelegate::DefaultWebContentsDelegate() {
+}
+
+DefaultWebContentsDelegate::~DefaultWebContentsDelegate() {
+}
+
+}