Add Blimp feedback data about visible BlimpContents.

blimp/client/core/contents/blimp_contents_manager.h

Index: blimp/client/core/contents/blimp_contents_manager.h
diff --git a/blimp/client/core/contents/blimp_contents_manager.h b/blimp/client/core/contents/blimp_contents_manager.h
index 07b922eb5e2dacd4aacc9366a20c8d1a2c856206..3e8bb5752c2ffe2151d6e50ee915ea908eadf234 100644
--- a/blimp/client/core/contents/blimp_contents_manager.h
+++ b/blimp/client/core/contents/blimp_contents_manager.h
@@ -6,6 +6,7 @@
 #define BLIMP_CLIENT_CORE_CONTENTS_BLIMP_CONTENTS_MANAGER_H_
 
 #include <map>
+#include <vector>
 
 #include "base/memory/weak_ptr.h"
 #include "blimp/client/core/contents/blimp_contents_impl.h"
@@ -44,6 +45,10 @@ class BlimpContentsManager {
   // The caller can query the contents through its id.
   BlimpContentsImpl* GetBlimpContents(int id);
 
+  // Returns a vector of the currently active BlimpContentsImpls. There is no
+  // guarantee for the lifetime of these pointers after this stack frame.
+  std::vector<BlimpContentsImpl*> GetAllActiveBlimpContents();
+
  private:
   class BlimpContentsDeletionObserver;
   friend class BlimpContentsDeletionObserver;