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/drawbitmaprect.cpp b/gm/drawbitmaprect.cpp
index 33a13f0..9c6fa3b 100644
--- a/gm/drawbitmaprect.cpp
+++ b/gm/drawbitmaprect.cpp
@@ -86,7 +86,7 @@
         return SkString("drawbitmaprect");
     }
 
-    SkISize onISize() { return make_isize(gSize, gSize); }
+    SkISize onISize() { return SkISize::Make(gSize, gSize); }
 
     virtual void onDraw(SkCanvas* canvas) {
         static const int kBmpSize = 2048;