make u32 check runtime

Bug: 848521
Change-Id: Id3deb30447cc5caa27203b46b6b257e76cd88679
Reviewed-on: https://skia-review.googlesource.com/140986
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Reed <reed@google.com>

include/private/SkArenaAlloc.h

@@ -134,7 +134,9 @@ public:
 private:
     static void AssertRelease(bool cond) { if (!cond) { ::abort(); } }
     static uint32_t ToU32(size_t v) {
-        assert(v <= 0xffffffff);
+#if SIZE_MAX > 0xffffffff
+        AssertRelease(v <= 0xffffffff);
+#endif
         return (uint32_t)v;
     }