Use class def index from java.lang.Class.

Bug: 10244719
This removes the computation of the dex file index, when necessary this is
computed by searching the dex file. Its only necessary in
dalvik.system.DexFile.defineClassNative and DexFile::FindInClassPath, the
latter not showing up significantly in profiling with this change.

(cherry-picked from 8b2c0b9abc3f520495f4387ea040132ba85cae69)
Change-Id: I20c73a3b17d86286428ab0fd21bc13f51f36c85c
diff --git a/runtime/mirror/proxy.h b/runtime/mirror/proxy.h
index 7c5bc39..18a84dc 100644
--- a/runtime/mirror/proxy.h
+++ b/runtime/mirror/proxy.h
@@ -25,6 +25,8 @@
 
 namespace mirror {
 
+// All proxy objects have a class which is a synthesized proxy class. The synthesized proxy class
+// has the static fields used to implement reflection on proxy objects.
 class MANAGED SynthesizedProxyClass : public Class {
  public:
   ObjectArray<Class>* GetInterfaces() {
@@ -41,6 +43,7 @@
   DISALLOW_IMPLICIT_CONSTRUCTORS(SynthesizedProxyClass);
 };
 
+// C++ mirror of java.lang.reflect.Proxy.
 class MANAGED Proxy : public Object {
  private:
   Object* h_;