remove dynamic_cast because rtti is disabled on some platforms (but not all??)

BUG=

Review URL: https://codereview.appspot.com/7102056

git-svn-id: http://skia.googlecode.com/svn/trunk@7161 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/utils/SkRTConf.cpp b/src/utils/SkRTConf.cpp
index fcc51c3..2ccfd06 100644
--- a/src/utils/SkRTConf.cpp
+++ b/src/utils/SkRTConf.cpp
@@ -267,8 +267,8 @@
     }
     
     for (SkRTConfBase **confBase = confArray->begin(); confBase != confArray->end(); confBase++) {
-        
-        SkRTConf<bool> *concrete = dynamic_cast<SkRTConf<bool> *>(*confBase);
+        // static_cast here is okay because there's only one kind of child class.
+        SkRTConf<bool> *concrete = static_cast<SkRTConf<bool> *>(*confBase);
         
         if (concrete) {
             concrete->set(value);