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/extractbitmap.cpp b/gm/extractbitmap.cpp
index 6e7409d..711a20a 100644
--- a/gm/extractbitmap.cpp
+++ b/gm/extractbitmap.cpp
@@ -37,7 +37,7 @@
     }
 
     virtual SkISize onISize() SK_OVERRIDE {
-        return make_isize(600, 600);
+        return SkISize::Make(600, 600);
     }
 
     virtual void onDraw(SkCanvas* canvas) SK_OVERRIDE {