ART: PathClassLoader for compiler

Use an actual PathClassLoader when compiling apps, instead of a
side structure and cutout.

This CL sets up a minimal object 'cluster' that recreates the Java
side of a regular ClassLoader such that the Class-Linker will
recognize it and use the internal native fast-path.

This CL removes the now unnecessary compile-time-classpath and
replaces it with a single 'compiling-the-boot-image' flag in the
compiler callbacks.

Note: This functionality is *only* intended for the compiler, as
the objects have not been completely initialized.

Bug: 19781184

Change-Id: I7f36af12dd7852d21281110a25c119e8c0669c1d
diff --git a/runtime/noop_compiler_callbacks.h b/runtime/noop_compiler_callbacks.h
index 300abc9..a40659e 100644
--- a/runtime/noop_compiler_callbacks.h
+++ b/runtime/noop_compiler_callbacks.h
@@ -23,7 +23,7 @@
 
 class NoopCompilerCallbacks FINAL : public CompilerCallbacks {
  public:
-  NoopCompilerCallbacks() {}
+  NoopCompilerCallbacks() : CompilerCallbacks(false) {}
   ~NoopCompilerCallbacks() {}
 
   bool MethodVerified(verifier::MethodVerifier* verifier ATTRIBUTE_UNUSED) OVERRIDE {