Clear a dangling reference from WebMIDIAccessor to MIDIAccessorClient

Implementations of WebMIDIAccessor in //content holds an raw pointer to
MIDIAccessorClient, however, the pointer may be invalid between GC marking
and its sweeping.

This CL removes the pointer by clearing WebMIDIAccessor itself on the
context destruction.

Bug: 846182
Change-Id: I857769a34b6c64e48b0540a117b29c94194ed611
Reviewed-on: https://chromium-review.googlesource.com/1071157Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561407}

third_party/blink/renderer/modules/webmidi/midi_access_initializer.cc

@@ -30,8 +30,11 @@ MIDIAccessInitializer::MIDIAccessInitializer(ScriptState* script_state,
                                              const MIDIOptions& options)
     : ScriptPromiseResolver(script_state), options_(options) {}
 
-void MIDIAccessInitializer::ContextDestroyed(ExecutionContext*) {
+void MIDIAccessInitializer::ContextDestroyed(ExecutionContext* context) {
+  accessor_.reset();
   permission_service_.reset();
+
+  ScriptPromiseResolver::ContextDestroyed(context);
 }
 
 ScriptPromise MIDIAccessInitializer::Start() {