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/texdata.cpp b/gm/texdata.cpp
index 72735a1..ca82292 100644
--- a/gm/texdata.cpp
+++ b/gm/texdata.cpp
@@ -31,7 +31,7 @@
     }
 
     virtual SkISize onISize() {
-        return make_isize(2*S, 2*S);
+        return SkISize::Make(2*S, 2*S);
     }
 
     virtual uint32_t onGetFlags() const SK_OVERRIDE { return kGPUOnly_Flag; }