Refactor some of the snapshot writer classes to eliminate code duplication.

vm/dart_api_impl.cc

Index: vm/dart_api_impl.cc
===================================================================
--- vm/dart_api_impl.cc	(revision 2716)
+++ vm/dart_api_impl.cc	(working copy)
@@ -650,7 +650,7 @@
   isolate->object_store()->set_root_library(Library::Handle());
   SnapshotWriter writer(Snapshot::kFull, buffer, ApiAllocator);
   writer.WriteFullSnapshot();
-  *size = writer.Size();
+  *size = writer.BytesWritten();
   return Api::Success();
 }
 
@@ -679,7 +679,7 @@
   }
   ScriptSnapshotWriter writer(buffer, ApiAllocator);
   writer.WriteScriptSnapshot(library);
-  *size = writer.Size();
+  *size = writer.BytesWritten();
   return Api::Success();
 }