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/tinybitmap.cpp b/gm/tinybitmap.cpp
index 92bf7c6..a188af9 100644
--- a/gm/tinybitmap.cpp
+++ b/gm/tinybitmap.cpp
@@ -40,7 +40,7 @@
         return SkString("tinybitmap");
     }
 
-    virtual SkISize onISize() { return make_isize(100, 100); }
+    virtual SkISize onISize() { return SkISize::Make(100, 100); }
 
     virtual void onDraw(SkCanvas* canvas) {
         SkBitmap bm = make_bitmap();