JDA: Fix deadlock for non-JDA case

The code will enter deadlock if the device has no JPEG decode
accelerator.
This CL fixes it.

BUG=chromium:790838
TEST=test camera preview on minnie

Change-Id: I695f80351a451614c7c830e3e31b40196fe9b457
Reviewed-on: https://chromium-review.googlesource.com/802935Reviewed-by: Ricky Liang <jcliang@chromium.org>
Reviewed-by: Max Morin <maxmorin@chromium.org>
Commit-Queue: Ricky Liang <jcliang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521085}

content/browser/renderer_host/media/video_capture_gpu_jpeg_decoder.cc

@@ -282,8 +282,8 @@ void VideoCaptureGpuJpegDecoder::FinishInitialization(
   TRACE_EVENT0("gpu", "VideoCaptureGpuJpegDecoder::FinishInitialization");
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
 
-  base::AutoLock lock(lock_);
   if (unbound_remote_decoder.is_valid()) {
+    base::AutoLock lock(lock_);
     decoder_ = base::MakeUnique<media::MojoJpegDecodeAccelerator>(
         BrowserThread::GetTaskRunnerForThread(BrowserThread::IO),
         std::move(unbound_remote_decoder));