Add helpers for logging mojom objects.

mojo/public/cpp/bindings/array.h

Index: mojo/public/cpp/bindings/array.h
diff --git a/mojo/public/cpp/bindings/array.h b/mojo/public/cpp/bindings/array.h
index c03da87a7fab388e9e54fc454af1a98e009c5033..bbf8e5c93cfa74ac10d3bc2ef25b68374ea13665 100644
--- a/mojo/public/cpp/bindings/array.h
+++ b/mojo/public/cpp/bindings/array.h
@@ -9,6 +9,7 @@
 
 #include <algorithm>
 #include <cstddef>
+#include <iosfwd>
 #include <set>
 #include <string>
 #include <vector>
@@ -309,6 +310,27 @@ struct TypeConverter<std::set<E>, Array<T>> {
   }
 };
 
+// Prints the contents of an array to an output stream for debugging purposes.
+template <typename T>
+std::ostream& operator<<(std::ostream& os, const mojo::Array<T>& array) {
+  if (array) {
+    os << "[";
+    bool first = true;
+    for (auto it = array.storage().cbegin(); it != array.storage().cend();
+         ++it) {
+      if (first)
+        first = false;
+      else
+        os << ", ";
+      os << *it;
+    }
+    os << "]";
+  } else {
+    os << "null";
+  }
+  return os;
+}
+
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_ARRAY_H_