Cleanup: Get rid of make_isize() function from gm.h.

This helper function is not necessary.

The same thing can be achieved by using SkISize::Make() provided by
SkTSize API.

BUG=skia:2645
TEST=make dm && out/Debug/dm
R=robertphillips@google.com, reed@google.com

Author: tfarina@chromium.org

Review URL: https://codereview.chromium.org/326523002
diff --git a/gm/matrixconvolution.cpp b/gm/matrixconvolution.cpp
index c081bf5..592489c 100644
--- a/gm/matrixconvolution.cpp
+++ b/gm/matrixconvolution.cpp
@@ -46,7 +46,7 @@
     }
 
     virtual SkISize onISize() {
-        return make_isize(500, 300);
+        return SkISize::Make(500, 300);
     }
 
     void draw(SkCanvas* canvas, int x, int y, const SkIPoint& kernelOffset,