debugger: Display index numbers for each op.
debugger: Display index numbers for each op.

Screenshot:
  https://screenshot.googleplex.com/KT88LSdNbPw.png

BUG=skia:

Review URL: https://codereview.chromium.org/1680563004

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot