For these two fields, an offset of 0 was used to mark non-existent
tables. After this CL, it always contains a real offset even if the
table is empty. This matches behavior of other embedded metadata
(constant pool and code comments). Remnants of the old behavior still
remain in WasmCode and HandlerTable.

Drive-by: Update comments describing Code object layout.
Drive-by: Unify naming of Code offset constants.

Bug: v8:8758
Change-Id: Ia8a1f66988b6c294a026b96f4f272fc5583a8c30
Reviewed-on: https://chromium-review.googlesource.com/c/1451880Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59360}