Check context is attached before creating MediaRecorder

Bug: 896736
Change-Id: I3ccfd2188fb15704af14c8af050e0a5667855d34
Reviewed-on: https://chromium-review.googlesource.com/c/1324231
Commit-Queue: Emircan Uysaler <emircan@chromium.org>
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606242}
ExecutionContext* MediaRecorder::GetExecutionContext() const {
  return PausableObject::GetExecutionContext();
}

ExecutionContext* MediaRecorder::GetExecutionContext() const {
  return PausableObject::GetExecutionContext();
}
