Fix TODO in SourceMapConsumerV3, as the GWT version of SourceMapObjec…

…t does return extensions.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=128538222

src/com/google/debugging/sourcemap/SourceMapConsumerV3.java

@@ -99,7 +99,8 @@ void parse(SourceMapObject sourceMapObject, SourceMapSupplier sectionSupplier)
       lines = new ArrayList<>();
     }
 
-    // TODO(thorogood): Note that when compiled with GWT, this may not contain anything.
+    // The value type of each extension is the native JSON type (e.g. JsonObject, or JSONObject
+    // when compiled with GWT).
     extensions.putAll(sourceMapObject.getExtensions());
     new MappingBuilder(sourceMapObject.getMappings()).build();
   }