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/internal_links.cpp b/gm/internal_links.cpp
index 3fdbbd5..7e5ce62 100644
--- a/gm/internal_links.cpp
+++ b/gm/internal_links.cpp
@@ -29,7 +29,7 @@
     }
 
     virtual SkISize onISize() {
-        return make_isize(700, 500);
+        return SkISize::Make(700, 500);
     }
 
     virtual void onDraw(SkCanvas* canvas) {