bindings: Pass dictionaries to callback methods by const reference

Before this CL, strings and container types were passed to callback
method by const reference. Dictionary types should also be passed
by const ref to avoid unnecessary copying.

BUG=557590

Review URL: https://codereview.chromium.org/1455883003

Cr-Commit-Position: refs/heads/master@{#360549}

third_party/WebKit/Source/bindings/scripts/v8_callback_interface.py

@@ -63,7 +63,8 @@ def cpp_type(idl_type):
         return 'void'
     # Callbacks use raw pointers, so raw_type=True
     raw_cpp_type = idl_type.cpp_type_args(raw_type=True)
-    if raw_cpp_type.startswith(('Vector', 'HeapVector', 'WillBeHeapVector')):
+    # Pass containers and dictionaries to callback method by const reference rather than by value
+    if raw_cpp_type.startswith(('Vector', 'HeapVector', 'WillBeHeapVector')) or idl_type.is_dictionary:
         return 'const %s&' % raw_cpp_type
     return raw_cpp_type