Add --matrix to DM, to play around with and maybe use on a bot.

BUG=skia:
R=reed@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/563723005
diff --git a/dm/DMGpuGMTask.cpp b/dm/DMGpuGMTask.cpp
index 3178fd6..65d65d9 100644
--- a/dm/DMGpuGMTask.cpp
+++ b/dm/DMGpuGMTask.cpp
@@ -1,5 +1,4 @@
 #include "DMGpuGMTask.h"
-
 #include "DMUtil.h"
 #include "DMWriteTask.h"
 #include "SkCommandLineFlags.h"
@@ -35,6 +34,7 @@
         return;
     }
     SkCanvas* canvas = surface->getCanvas();
+    CanvasPreflight(canvas);
 
     canvas->concat(fGM->getInitialTransform());
     fGM->draw(canvas);