Don't expose GetFunction on the scope iterator. Simply take it into account for GetFunctionDebugName

This is a step towards avoiding materializing function_ altogether if we deoptimize. Typically we only need the SharedFunctionInfo.

Cq-Include-Trybots: luci.chromium.try:linux_chromium_headless_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Idee78f02d1afe3d2cb70e93a6d96a5a33907f892
Reviewed-on: https://chromium-review.googlesource.com/1100474
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53789}