Avoid copying and strlen(3) in the FindClass path.

Change-Id: I789f3c883596d1852a2c1954ce7a207e6f937117
diff --git a/src/utils.cc b/src/utils.cc
index ced68de..bfc2272 100644
--- a/src/utils.cc
+++ b/src/utils.cc
@@ -268,10 +268,10 @@
   return descriptor;
 }
 
-std::string DescriptorToDot(const std::string& descriptor) {
+std::string DescriptorToDot(const StringPiece& descriptor) {
   DCHECK_EQ(descriptor[0], 'L');
   DCHECK_EQ(descriptor[descriptor.size()-1], ';');
-  std::string dot(descriptor.substr(1, descriptor.size() - 2));
+  std::string dot(descriptor.substr(1, descriptor.size() - 2).ToString());
   std::replace(dot.begin(), dot.end(), '/', '.');
   return dot;
 }