Make debugger raster window background black

Make debugger raster window background black. This helps to understand
where the picture boundaries are. It is probably more common for pictures to
have non-black background vs non-white background.

This makes raster window similar to opengl window.

Review URL: https://codereview.chromium.org/816363002
diff --git a/debugger/QT/SkRasterWidget.cpp b/debugger/QT/SkRasterWidget.cpp
index ad05d39..7de5ac4 100644
--- a/debugger/QT/SkRasterWidget.cpp
+++ b/debugger/QT/SkRasterWidget.cpp
@@ -14,7 +14,7 @@
     fDevice = new SkBitmapDevice(fBitmap);
     fDebugger = debugger;
     fCanvas = new SkCanvas(fDevice);
-    this->setStyleSheet("QWidget {background-color: white; border: 1px solid #cccccc;}");
+    this->setStyleSheet("QWidget {background-color: black; border: 1px solid #cccccc;}");
 }
 
 SkRasterWidget::~SkRasterWidget() {