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

Change-Id: I789f3c883596d1852a2c1954ce7a207e6f937117
diff --git a/src/object.cc b/src/object.cc
index 10d4289..b98235b 100644
--- a/src/object.cc
+++ b/src/object.cc
@@ -307,15 +307,15 @@
     ++p; // Either the ';' or the primitive type.
 
     std::string descriptor(start, (p - start));
-    return class_linker->FindClass(descriptor, cl);
+    return class_linker->FindClass(descriptor.c_str(), cl);
   } else if (*p == 'L') {
     const char* start = p;
     while (*p != ';') {
       ++p;
     }
     ++p;
-    StringPiece descriptor(start, (p - start));
-    return class_linker->FindClass(descriptor.ToString(), cl);
+    std::string descriptor(start, (p - start));
+    return class_linker->FindClass(descriptor.c_str(), cl);
   } else {
     return class_linker->FindPrimitiveClass(*p++);
   }