Revert "Revert "IWYU""

This reverts commit 3c727d2386059c1d7cbdcdc9bef5fa18ed33667e.

BUG=skia:

Change-Id: I72e9b40fec96d044e0ac12906669a52529e77882
Reviewed-on: https://skia-review.googlesource.com/8356
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/gm/drawregion.cpp b/gm/drawregion.cpp
index a2633f9..2230f09 100644
--- a/gm/drawregion.cpp
+++ b/gm/drawregion.cpp
@@ -7,6 +7,7 @@
 
 #include "gm.h"
 #include "SkCanvas.h"
+#include "SkRegion.h"
 
 /**
  *  This is very similar to the RectGrid macrobench in Android.