ART: Expose vreg and visitor in RootInfo

Expose the register and visitor in JavaFrameRootInfo, to be able
to investigate a root programmatically.

Also mark StackVisitor::GetFrameDepth() as a const function.

Bug: 31385354
Test: m test-art-host
Change-Id: Ie05d2cfe3f4e9def89311bb18bd0c0bbf1a45625
1 file changed