If the imported wasm function is being debugged (i.e. redirects to the
interpreter), call it via the JS_TO_WASM stub, such that we can disable
the breakpoint later by patching the exported function.

This also contains a drive-by fix in wasm-translation.cc (for the case
that all known positions are bigger than the requested one).

R=titzer@chromium.org, kozyatinskiy@chromium.org
BUG=v8:5971, v8:5822

Review-Url: https://codereview.chromium.org/2720813002
Cr-Commit-Position: refs/heads/master@{#43583}