This CL renames field offsets from *Index to *Offset in the
StackFrameInfo class for consistency.

R=mstarzinger@chromium.org

Bug: v8:8562
Change-Id: I9ccec3e6f924a4040d26dc677fdd1ec620cf0463
Reviewed-on: https://chromium-review.googlesource.com/c/1460937Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59460}