[Fuchsia] Destroy audio decryptor on the right thread

Previously FuchsiaDecryptor was destroying FuchsiaClearStreamDecryptor
in the destructor which may run on a thread that's different than the
thread on which FuchsiaClearStreamDecryptor was created. Updated
FuchsiaDecryptor destructor to delete audio_decryptor_ asynchronously
on the right thread.

Bug: 1047783
Change-Id: I9c4dc5d7a292078eb632c0c44be6a68cc949b7e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2033813
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738277}

media/fuchsia/cdm/fuchsia_decryptor.cc

@@ -5,7 +5,9 @@
 #include "media/fuchsia/cdm/fuchsia_decryptor.h"
 
 #include "base/fuchsia/fuchsia_logging.h"
+#include "base/location.h"
 #include "base/logging.h"
+#include "base/threading/thread_task_runner_handle.h"
 #include "media/base/decoder_buffer.h"
 #include "media/base/video_frame.h"
 #include "media/fuchsia/cdm/fuchsia_stream_decryptor.h"
@@ -18,7 +20,12 @@ FuchsiaDecryptor::FuchsiaDecryptor(
   DCHECK(cdm_);
 }
 
-FuchsiaDecryptor::~FuchsiaDecryptor() = default;
+FuchsiaDecryptor::~FuchsiaDecryptor() {
+  if (audio_decryptor_) {
+    audio_decryptor_task_runner_->DeleteSoon(FROM_HERE,
+                                             std::move(audio_decryptor_));
+  }
+}
 
 void FuchsiaDecryptor::RegisterNewKeyCB(StreamType stream_type,
                                         const NewKeyCB& new_key_cb) {
@@ -37,8 +44,10 @@ void FuchsiaDecryptor::Decrypt(StreamType stream_type,
     return;
   }
 
-  if (!audio_decryptor_)
+  if (!audio_decryptor_) {
+    audio_decryptor_task_runner_ = base::ThreadTaskRunnerHandle::Get();
     audio_decryptor_ = FuchsiaClearStreamDecryptor::Create(cdm_);
+  }
 
   audio_decryptor_->Decrypt(std::move(encrypted), decrypt_cb);
 }

media/fuchsia/cdm/fuchsia_decryptor.h

@@ -7,6 +7,8 @@
 
 #include <memory>
 
+#include "base/memory/ref_counted.h"
+#include "base/single_thread_task_runner.h"
 #include "base/synchronization/lock.h"
 #include "base/thread_annotations.h"
 #include "media/base/decryptor.h"
@@ -60,6 +62,9 @@ class FuchsiaDecryptor : public Decryptor {
 
   std::unique_ptr<FuchsiaClearStreamDecryptor> audio_decryptor_;
 
+  // TaskRunner for the thread on which |audio_decryptor_| was created.
+  scoped_refptr<base::SingleThreadTaskRunner> audio_decryptor_task_runner_;
+
   DISALLOW_COPY_AND_ASSIGN(FuchsiaDecryptor);
 };