[PTZ] Update fake device stream when pan and tilt change

This CL makes sure the fake device stream gets updated when pan and tilt
options change so that web developers get to see effects in real-time.

Bug: 934063
Change-Id: Idaddacdbc22f1eb77a3b4e2fb9546c33e37f2fcb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2276424Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#784474}

media/capture/video/fake_video_capture_device.cc

@@ -432,8 +432,14 @@ void PacmanFramePainter::DrawPacman(base::TimeDelta elapsed_time,
   SkCanvas canvas(bitmap);
 
   const SkScalar unscaled_zoom = fake_device_state_->zoom / 100.f;
+  const SkScalar translate_x =
+      (fake_device_state_->pan - kMinPan) * (width / (kMaxPan - kMinPan));
+  const SkScalar translate_y =
+      (fake_device_state_->tilt - kMinTilt) * (height / (kMaxTilt - kMinTilt));
   SkMatrix matrix;
   matrix.setScale(unscaled_zoom, unscaled_zoom, width / 2, height / 2);
+  matrix.setTranslateX(translate_x);
+  matrix.setTranslateY(translate_y);
   canvas.setMatrix(matrix);
 
   // For the SK_N32 case, match the green color tone produced by the