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/convexpolyclip.cpp b/gm/convexpolyclip.cpp
index edafea2..6de72dc 100644
--- a/gm/convexpolyclip.cpp
+++ b/gm/convexpolyclip.cpp
@@ -88,7 +88,7 @@
         if (kBench_Mode != this->getMode()) {
             w *= 2;
         }
-        return make_isize(w, 540);
+        return SkISize::Make(w, 540);
     }
 
     virtual void onOnceBeforeDraw() SK_OVERRIDE {