Clarify heap write protection interface and use WritableVMIsolateScope to resolve issue 23217.

runtime/vm/pages.h

Index: runtime/vm/pages.h
diff --git a/runtime/vm/pages.h b/runtime/vm/pages.h
index e67bc812f0e7c842336d5c48bb3c1a3d5609db25..f7acf90e0d7f516fbc752d51518132b6fb0a2391 100644
--- a/runtime/vm/pages.h
+++ b/runtime/vm/pages.h
@@ -277,7 +277,8 @@ class PageSpace {
         (ExternalInWords() > max_external_in_words_);
   }
 
-  // TODO(koda): Unify protection handling.
+  // Note: Code pages are made executable/non-executable when 'read_only' is
+  // true/false, respectively.
   void WriteProtect(bool read_only);
   void WriteProtectCode(bool read_only);