Mac QTKit video capture: Correct DCHECK #inputs==1 location

Gosh the DCHECK for #inputs == 1 was located _after_ removing the
input XD

Interesting that no one else hit this problem, including myself.

BUG=407046

Review URL: https://codereview.chromium.org/498213002

Cr-Commit-Position: refs/heads/master@{#291742}

media/video/capture/mac/video_capture_device_qtkit_mac.mm

@@ -236,8 +236,8 @@
 
 - (void)stopCaptureOnUIThread:(id)dummy {
   if ([[captureSession_ inputs] count] > 0) {
-    [captureSession_ removeInput:captureDeviceInput_];
     DCHECK_EQ([[captureSession_ inputs] count], 1u);
+    [captureSession_ removeInput:captureDeviceInput_];
     [captureSession_ stopRunning];
   }
   if ([[captureSession_ outputs] count] > 0) {