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/bitmapmatrix.cpp b/gm/bitmapmatrix.cpp
index b5b8e59..052d153 100644
--- a/gm/bitmapmatrix.cpp
+++ b/gm/bitmapmatrix.cpp
@@ -32,7 +32,7 @@
         return SkString("drawbitmapmatrix");
     }
 
-    virtual SkISize onISize() SK_OVERRIDE { return make_isize(1024, 256); }
+    virtual SkISize onISize() SK_OVERRIDE { return SkISize::Make(1024, 256); }
 
     virtual void onDraw(SkCanvas* canvas) SK_OVERRIDE {
         SkBitmap bm;