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/shadertext.cpp b/gm/shadertext.cpp
index 4936801..ac85846 100644
--- a/gm/shadertext.cpp
+++ b/gm/shadertext.cpp
@@ -108,7 +108,7 @@
         return SkString("shadertext");
     }
 
-    SkISize onISize() { return make_isize(1450, 500); }
+    SkISize onISize() { return SkISize::Make(1450, 500); }
 
     virtual void onDraw(SkCanvas* canvas) {
         const char text[] = "Shaded Text";