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/selftest.cpp b/gm/selftest.cpp
index dd6e67b..a332c5d 100644
--- a/gm/selftest.cpp
+++ b/gm/selftest.cpp
@@ -26,7 +26,7 @@
         return fName;
     }
 
-    SkISize onISize() { return skiagm::make_isize(kWidth, kHeight); }
+    SkISize onISize() { return SkISize::Make(kWidth, kHeight); }
 
     virtual void onDraw(SkCanvas* canvas) {
         SkPaint paint;