Use correct handle scope offset from StackVisitor

Incorrect offset resulted in bad GC roots due to invalid
Stack::GetThisObject for native methods.

Bug: 19070497
Bug: 18785293

(cherry picked from commit e4b7c892c4f40e76c172a77069afde3fe5ce87da)
Change-Id: I10e144cffac00978e3c84d43a30caccd61559b27
2 files changed