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/aarectmodes.cpp b/gm/aarectmodes.cpp
index ee0bace..b2a28c1 100644
--- a/gm/aarectmodes.cpp
+++ b/gm/aarectmodes.cpp
@@ -141,7 +141,7 @@
             return SkString("aarectmodes");
         }
 
-        virtual SkISize onISize() { return make_isize(640, 480); }
+        virtual SkISize onISize() { return SkISize::Make(640, 480); }
 
         virtual void onDraw(SkCanvas* canvas) {
             if (false) { // avoid bit rot, suppress warning