ART: Some runtime cleanup

Use an enum for the compiler-callback mode.

Refactor and remove some unnecessary includes in runtime.h.

Change-Id: If2245fa470171311b8e05b677cf6bb28f209585a
diff --git a/runtime/mirror/art_field.cc b/runtime/mirror/art_field.cc
index 5d543a1..4c36753 100644
--- a/runtime/mirror/art_field.cc
+++ b/runtime/mirror/art_field.cc
@@ -45,7 +45,7 @@
 void ArtField::SetOffset(MemberOffset num_bytes) {
   DCHECK(GetDeclaringClass()->IsLoaded() || GetDeclaringClass()->IsErroneous());
   if (kIsDebugBuild && Runtime::Current()->IsAotCompiler() &&
-      Runtime::Current()->GetCompilerCallbacks()->IsBootImage()) {
+      Runtime::Current()->IsCompilingBootImage()) {
     Primitive::Type type = GetTypeAsPrimitiveType();
     if (type == Primitive::kPrimDouble || type == Primitive::kPrimLong) {
       DCHECK_ALIGNED(num_bytes.Uint32Value(), 8);