This CL ads a StartPosition and EndPosition accessors on SFI and
ScopeInfo to facilitate future refactoring. In a future CL the
start and end position are no longer stored directly on SFIs.

This CL will temporarily increase memory since the position info
is duplicated on the SFI and the ScopeInfo.

Drive-by-fix: Clean up some constants in ScopeInfo

Bug: v8:7066
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I1a8c5dd4e2156c007e04d92e72e478b915516e0d
Reviewed-on: https://chromium-review.googlesource.com/955629Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51987}