Simplify Class::IsArtFieldClass().

Fix the slight glitch that when ImageSpace::VerifyImageAllocations()
called in ImageSpace::Create(), the ArtField and ArtMethod class roots
weren't set, which were used by DCHECKs in Object::Size(), which
VerifyImageAllocations() calls, by delaying the point of the
VerifyImageAllocations() call to Runtime::Init() at which point the
class linker has set the class roots.

To completely disable read barriers from Object::SizeOf(), the
ReadBarrierOption template parameter should have been added to
Class::GetInstanceField(), which calls GetFieldObject(), when it's
called from Class::IsArtFieldClass(). This change fixes this by
removing the need for the call, instead of adding the
ReadBarrierOption parameter.

Bug: 12687968
Change-Id: Ibbecc08f4e3b898851805d690dff8ccac55e94f2