Mac Capture: Never request PIXEL_FORMAT_MJPEG AVCaptureVideoDataOutput

Prior to crrev.com/804584, the code to select AVCaptureVideoDataOutput's
pixel format looked like it might select PIXEL_FORMAT_MJPEG (aka dmb1
aka kCMVideoCodecType_JPEG_OpenDML), but it actually would always select
PIXEL_FORMAT_YUY2 (aka yuv2 aka kCMPixelFormat_422YpCbCr8).

And it turns out that PIXEL_FORMAT_MJPEG doesn't work. So let's make
sure we don't select it.

There remains a hunch that MJPEG will be the most efficient capture
format, and we should evaluate doing the decode and conversion
ourselves.

Bug: 1124884
Change-Id: I04473b117c05ccadbdd3226badac20656d6241a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2399238Reviewed-by: Miguel Casas <mcasas@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805022}

media/capture/video/mac/video_capture_device_avfoundation_mac.mm

@@ -435,8 +435,11 @@ AVCaptureDeviceFormat* FindBestCaptureFormat(
   }
 
   if (best_fourcc == kCMVideoCodecType_JPEG_OpenDML) {
-    [_captureSession removeOutput:_stillImageOutput];
-    _stillImageOutput.reset();
+    // Capturing MJPEG directly never worked. Request a conversion to what has
+    // historically been the default pixel format.
+    // TODO(https://crbug.com/1124884): Investigate the performance of
+    // performing MJPEG ourselves.
+    best_fourcc = kCMPixelFormat_422YpCbCr8;
   }
 
   // The capture output has to be configured, despite Mac documentation