[Image Capture, Android] Fix for "Check failed: false. |callback_id| not found"

Fixes a race condition that can lead to a crash when taking a photo via the
image capture web API on Android.

Bug: 846576
Change-Id: I0008391acfd4fd1f84db1d403d33c0dc31410294
Reviewed-on: https://chromium-review.googlesource.com/1117845Reviewed-by: Emircan Uysaler <emircan@chromium.org>
Commit-Queue: Christian Fremerey <chfremer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571538}

media/capture/video/android/video_capture_device_android.cc

@@ -466,11 +466,13 @@ void VideoCaptureDeviceAndroid::DoTakePhoto(TakePhotoCallback callback) {
   std::unique_ptr<TakePhotoCallback> heap_callback(
       new TakePhotoCallback(std::move(callback)));
   const intptr_t callback_id = reinterpret_cast<intptr_t>(heap_callback.get());
-  if (!Java_VideoCapture_takePhoto(env, j_capture_, callback_id))
-    return;
 
-  {
+  // We need lock here because asynchronous response to
+  // Java_VideoCapture_takePhoto(), i.e. a call to OnPhotoTaken, arrives from a
+  // separate thread, and it can arrive before |photo_callbacks_.push_back()|
+  // has executed.
   base::AutoLock lock(photo_callbacks_lock_);
+  if (Java_VideoCapture_takePhoto(env, j_capture_, callback_id)) {
     photo_callbacks_.push_back(std::move(heap_callback));
   }
 }