Enable Oilpan by default for webaudio/

Source/platform/audio/AudioDSPKernel.h

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 49 matching lines @@
     float sampleRate() const { return m_sampleRate; }
     double nyquist() const { return 0.5 * sampleRate(); }
 
     AudioDSPKernelProcessor* processor() { return m_kernelProcessor; }
     const AudioDSPKernelProcessor* processor() const { return m_kernelProcessor; }
 
     virtual double tailTime() const = 0;
     virtual double latencyTime() const = 0;
 
 protected:
+    // Oilpan: This raw pointer is safe because the AudioDSPKernelProcessor
+    // object is guaranteed to be kept alive while the AudioDSPKernel object
+    // is alive. Ideally we want to move the AudioDSPKernel hierarchy to
+    // the heap and use a Member<AudioDSPKernelProcessor>. However, we cannot
+    // do that because AudioDSPKernel can be allocated in audio threads
+    // (which are not registered to Oilpan).
+    GC_PLUGIN_IGNORE("http://crbug.com/404578")
     AudioDSPKernelProcessor* m_kernelProcessor;
     float m_sampleRate;
 };
 
 } // namespace blink
 
 #endif // AudioDSPKernel_h