Introduce AudioSourceProviderClientLockScope.

Source/platform/audio/AudioSourceProviderClient.h

Index: Source/platform/audio/AudioSourceProviderClient.h
diff --git a/Source/platform/audio/AudioSourceProviderClient.h b/Source/platform/audio/AudioSourceProviderClient.h
index 2ef400eca0c416d7e4343045d5a0ab89165c44f0..c6faefaf12c47f3e8da1be831e71cee9c7638a5d 100644
--- a/Source/platform/audio/AudioSourceProviderClient.h
+++ b/Source/platform/audio/AudioSourceProviderClient.h
@@ -32,6 +32,7 @@ namespace blink {
 class AudioSourceProviderClient : public WillBeGarbageCollectedMixin {
 public:
     virtual void setFormat(size_t numberOfChannels, float sampleRate) = 0;
+    // Oilpan: Callers should keep this object alive during lock() and unlock().
     virtual void lock() { }
     virtual void unlock() { }
     virtual void trace(Visitor*) { }