Expose the internal HeapHashMap for the parameter iteration

For easier iteration of stored AudioParam in the AudioParamMap,
this CL exposes the HeapHashMap (const) via a getter method. This is
a part of the graph event reporting POC CL:
https://chromium-review.googlesource.com/c/chromium/src/+/1696004

Additionally two unused methods are removed because now the customer
can directly access the map with the getter.

Bug: 970898
Change-Id: I8932d353fcbf8258f1364a741d46da2d6314afc0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1716092Reviewed-by: Raymond Toy <rtoy@chromium.org>
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680222}

third_party/blink/renderer/modules/webaudio/audio_param_map.h

@@ -22,14 +22,14 @@ class AudioParamMap final : public ScriptWrappable,
   DEFINE_WRAPPERTYPEINFO();
 
  public:
-  explicit AudioParamMap(
-      const HeapHashMap<String, Member<AudioParam>>& parameter_map);
+  using MapType = HeapHashMap<String, Member<AudioParam>>;
+
+  explicit AudioParamMap(const MapType& parameter_map);
 
   // IDL attributes / methods
   uint32_t size() const { return parameter_map_.size(); }
 
-  AudioParam* At(String name) { return parameter_map_.at(name); }
-  bool Contains(String name) { return parameter_map_.Contains(name); }
+  const MapType& GetHashMap() const { return parameter_map_; }
 
   void Trace(blink::Visitor* visitor) override {
     visitor->Trace(parameter_map_);
@@ -45,7 +45,7 @@ class AudioParamMap final : public ScriptWrappable,
                    AudioParam*&,
                    ExceptionState&) override;
 
-  const HeapHashMap<String, Member<AudioParam>> parameter_map_;
+  const MapType parameter_map_;
 };
 
 }  // namespace blink