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/points.cpp b/gm/points.cpp
index f4df741..793e2e8 100644
--- a/gm/points.cpp
+++ b/gm/points.cpp
@@ -24,7 +24,7 @@
     }
 
     virtual SkISize onISize() {
-        return make_isize(640, 490);
+        return SkISize::Make(640, 490);
     }
 
     static void fill_pts(SkPoint pts[], size_t n, SkLCGRandom* rand) {