Obtain graph/process lock when nullifying the buffer in Reverb

When the buffer is set to `null` while there is an active buffer
within a reverb object, SetBuffer() function can prematurely
nullify the `reverb_` and `shared_buffer_` while it is still
being accessed by the rendering thread.

This CL adds two locks (graph lock and process lock) when the
buffer gets nullified to ensure the synchronization between
two threads.

Change-Id: I8f501b6a16b3c7e16db767e0b279a1a53d6eb290
Bug: 1019226
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1888103
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710627}

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

@@ -103,6 +103,8 @@ void ConvolverHandler::SetBuffer(AudioBuffer* buffer,
   DCHECK(IsMainThread());
 
   if (!buffer) {
+    BaseAudioContext::GraphAutoLocker context_locker(Context());
+    MutexLocker locker(process_lock_);
     reverb_.reset();
     shared_buffer_ = nullptr;
     return;