For on-heap code, there is the nice feature of highlighting the current
pc on the "jco" gdb macro (calling {Code->Print}, {Code->Disassemble}
or {WasmCode::Disassemble}). For wasm code, this feature was missing so
far. This CL adds it.

R=ahaas@chromium.org

Change-Id: I0ee86d3c5cf9f42581f03c2ba4ec16b4c992e016
Reviewed-on: https://chromium-review.googlesource.com/1021517Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52721}