Pepper: Access PepperMediaDeviceManager through a WeakPtr

Its lifetime is scoped to the RenderFrame, and it might go away before the
hosts that refer to it.

BUG=423030

Review URL: https://codereview.chromium.org/653243003

Cr-Commit-Position: refs/heads/master@{#299897}
void PepperMediaDeviceManager::OnDeviceOpenFailed(int request_id) {
  NotifyDeviceOpened(request_id, false, std::string());
}

void PepperMediaDeviceManager::OnDeviceOpenFailed(int request_id) {
  NotifyDeviceOpened(request_id, false, std::string());
}
