Fix Wunused-but-set-variable warnings

Change-Id: I0e093fd35b11e9a765ef9c09f3b6346086ff66bf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/435983
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/gm/rasterhandleallocator.cpp b/gm/rasterhandleallocator.cpp
index 340b033..fa4488b 100644
--- a/gm/rasterhandleallocator.cpp
+++ b/gm/rasterhandleallocator.cpp
@@ -255,7 +255,7 @@
 
         RECT clip_bounds_RECT = toRECT(clip_bounds);
         HRGN hrgn = CreateRectRgnIndirect(&clip_bounds_RECT);
-        int result = SelectClipRgn(hdc, hrgn);
+        SK_MAYBE_UNUSED int result = SelectClipRgn(hdc, hrgn);
         SkASSERT(result != ERROR);
         result = DeleteObject(hrgn);
         SkASSERT(result != 0);