Various debug ART GC performance improvements

Removed unnecessary read barriers in SizeOf and a few other places.
Disabled disallow read barrier check.

Before:
GC time: 15.817s
Real 0m26.113s
user  1m16.780s
sys 0m3.152s

After:
GC time: 9.212s
real  0m19.875s
user  1m9.916s
sys 0m1.916s

Bug: 35644369
Test: test-art-host

Change-Id: I79a65259deff2a478a96e02ae69b14730b6dcbe6
diff --git a/runtime/mirror/object-inl.h b/runtime/mirror/object-inl.h
index 811f1ea..f83645e 100644
--- a/runtime/mirror/object-inl.h
+++ b/runtime/mirror/object-inl.h
@@ -513,8 +513,11 @@
   return GetClass<kVerifyFlags>()->IsPhantomReferenceClass();
 }
 
-template<VerifyObjectFlags kVerifyFlags, ReadBarrierOption kReadBarrierOption>
+template<VerifyObjectFlags kVerifyFlags>
 inline size_t Object::SizeOf() {
+  // Read barrier is never required for SizeOf since objects sizes are constant. Reading from-space
+  // values is OK because of that.
+  static constexpr ReadBarrierOption kReadBarrierOption = kWithoutReadBarrier;
   size_t result;
   constexpr auto kNewFlags = static_cast<VerifyObjectFlags>(kVerifyFlags & ~kVerifyThis);
   if (IsArrayInstance<kVerifyFlags, kReadBarrierOption>()) {