Prevent log creation in detached context

This CL fixes a race condition in which a MediaLog could be created in
a context that is already destroyed. See the attached bug for more
details.

Bug: 1154468
Change-Id: I26ae0c621d9a4557d224ca77f696ad3ce53b41c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2580478Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Auto-Submit: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835842}

third_party/blink/renderer/modules/webcodecs/codec_logger.cc

@@ -17,9 +17,18 @@ CodecLogger::CodecLogger(
     ExecutionContext* context,
     scoped_refptr<base::SingleThreadTaskRunner> task_runner) {
   DCHECK(context);
-  parent_media_log_ = Platform::Current()->GetMediaLog(
-      MediaInspectorContextImpl::From(*context), task_runner);
 
+  // Owners of |this| should be ExecutionLifeCycleObservers, and should call
+  // Neuter() if |context| is destroyed. The MediaInspectorContextImpl must
+  // outlive |parent_media_log_|. If |context| is already destroyed, owners
+  // might never call Neuter(), and MediaInspectorContextImpl* could be garbage
+  // collected before |parent_media_log_| is destroyed.
+  if (!context->IsContextDestroyed()) {
+    parent_media_log_ = Platform::Current()->GetMediaLog(
+        MediaInspectorContextImpl::From(*context), task_runner);
+  }
+
+  // NullMediaLog silently and safely does nothing.
   if (!parent_media_log_)
     parent_media_log_ = std::make_unique<media::NullMediaLog>();