Web MIDI: introduce MidiManager::Shutdown to shutdown gracefully

Platform dependent MidiManager implementations allocate extra threads
lazily on the IO thread, but it does not have a chance to stop them
correctly on the same IO thread, and stop it on the main thread.

This patch introduces Shutdown() method to be called before the IO
thread stopped so that the MidiManager implementations can stop the
extra threads gracefully on the right thread.

Platform specific fixes follow this change.

BUG=374341

Review URL: https://codereview.chromium.org/1315793008

Cr-Commit-Position: refs/heads/master@{#352564}

1 files changed, 12 insertions, 5 deletions

diff --git a/content/browser/media/midi_host.cc b/content/browser/media/midi_host.cc
index c26a719..bc697ed 100644
--- a/content/browser/media/midi_host.cc
+++ b/content/browser/media/midi_host.cc
@@ -56,12 +56,12 @@ MidiHost::MidiHost(int renderer_process_id,
       sent_bytes_in_flight_(0),
       bytes_sent_since_last_acknowledgement_(0),
       output_port_count_(0) {
-  CHECK(midi_manager_);
+  DCHECK(midi_manager_);
 }
 
 MidiHost::~MidiHost() {
   // Close an open session, or abort opening a session.
-  if (is_session_requested_)
+  if (is_session_requested_ && midi_manager_)
     midi_manager_->EndSession(this);
 }
 
@@ -84,7 +84,8 @@ bool MidiHost::OnMessageReceived(const IPC::Message& message) {
 
 void MidiHost::OnStartSession() {
   is_session_requested_ = true;
-  midi_manager_->StartSession(this);
+  if (midi_manager_)
+    midi_manager_->StartSession(this);
 }
 
 void MidiHost::OnSendData(uint32 port,
@@ -122,12 +123,14 @@ void MidiHost::OnSendData(uint32 port,
       return;
     sent_bytes_in_flight_ += data.size();
   }
-  midi_manager_->DispatchSendMidiData(this, port, data, timestamp);
+  if (midi_manager_)
+    midi_manager_->DispatchSendMidiData(this, port, data, timestamp);
 }
 
 void MidiHost::OnEndSession() {
   is_session_requested_ = false;
-  midi_manager_->EndSession(this);
+  if (midi_manager_)
+    midi_manager_->EndSession(this);
 }
 
 void MidiHost::CompleteStartSession(media::midi::Result result) {
@@ -216,6 +219,10 @@ void MidiHost::AccumulateMidiBytesSent(size_t n) {
   }
 }
 
+void MidiHost::Detach() {
+  midi_manager_ = nullptr;
+}
+
 // static
 bool MidiHost::IsValidWebMIDIData(const std::vector<uint8>& data) {
   bool in_sysex = false;