Remove the SharedFunctionInfo code field, inferring the code object
from the function_data field instead. In some cases, the function_data
field can now hold a Code object (e.g. some WASM cases).

(Reland of https://chromium-review.googlesource.com/952452)

TBR=mstarzinger@chromium.org

Bug: chromium:783853
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I10ea5be7ceed1b51362a2fad9be7397624d69343
Reviewed-on: https://chromium-review.googlesource.com/970649
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52136}