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/bitmapcopy.cpp b/gm/bitmapcopy.cpp
index b441e34..eaad8d0 100644
--- a/gm/bitmapcopy.cpp
+++ b/gm/bitmapcopy.cpp
@@ -57,7 +57,7 @@
     }
 
     virtual SkISize onISize() {
-        return make_isize(540, 330);
+        return SkISize::Make(540, 330);
     }
 
     virtual void onDraw(SkCanvas* canvas) {