[debug][cleanup] Use consistent StepInto and StepOver naming.

In the Chrome DevTools Protocol, the step actions are named StepOut,
StepOver, and StepInto, but internally we used StepOut, StepNext, and
StepIn instead. This change adjusts the naming to be consistent.

Bug: chromium:901814, chromium:1162229
Change-Id: Id3502a1b0a4aadd94734ec3d1fef73c1782fa220
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2928510


Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74877}