Compute baseLatency at construction and cache the value

Since the baseLatency value is fixed after construction, compute the
value when the AudioContext is constructed and cache the value so that
any queries just return the value from the slot.

Bug: 1005150
Change-Id: I0a4b8c4fb42deb5721f0ecdce5a8bc3b3c4cb26e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1903750Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713643}

third_party/blink/renderer/modules/webaudio/audio_context.cc

@@ -155,6 +155,20 @@ AudioContext::AudioContext(Document& document,
   }
 
   Initialize();
+
+  // Compute the base latency now and cache the value since it doesn't change
+  // once the context is constructed.  We need the destination to be initialized
+  // so we have to compute it here.
+  //
+  // TODO(hongchan): Due to the incompatible constructor between
+  // AudioDestinationNode and RealtimeAudioDestinationNode, casting directly
+  // from |destination()| is impossible. This is a temporary workaround until
+  // the refactoring is completed.
+  RealtimeAudioDestinationHandler& destination_handler =
+      static_cast<RealtimeAudioDestinationHandler&>(
+          destination()->GetAudioDestinationHandler());
+  base_latency_ = destination_handler.GetFramesPerBuffer() /
+                  static_cast<double>(sampleRate());
 }
 
 void AudioContext::Uninitialize() {
@@ -359,15 +373,7 @@ double AudioContext::baseLatency() const {
   DCHECK(IsMainThread());
   DCHECK(destination());
 
-  // TODO(hongchan): Due to the incompatible constructor between
-  // AudioDestinationNode and RealtimeAudioDestinationNode, casting directly
-  // from |destination()| is impossible. This is a temporary workaround until
-  // the refactoring is completed.
-  RealtimeAudioDestinationHandler& destination_handler =
-      static_cast<RealtimeAudioDestinationHandler&>(
-          destination()->GetAudioDestinationHandler());
-  return destination_handler.GetFramesPerBuffer() /
-         static_cast<double>(sampleRate());
+  return base_latency_;
 }
 
 MediaElementAudioSourceNode* AudioContext::createMediaElementSource(

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

@@ -176,6 +176,9 @@ class MODULES_EXPORT AudioContext : public BaseAudioContext {
   // Represents whether a context is suspended by explicit |context.suspend()|.
   bool suspended_by_user_ = false;
 
+  // baseLatency for this context
+  double base_latency_ = 0;
+
   // AudioContextManager for reporting audibility.
   mojo::Remote<mojom::blink::AudioContextManager> audio_context_manager_;