This fixes a bug where an exported function is being specialized, but
the callsite inside the JS_TO_WASM function was patched to call an
interpreter entry instead. We would not identify the call site as the
one to be patched during specialization, and would thus fail a DCHECK.

R=ahaas@chromium.org
BUG=v8:5822, chromium:702839

Change-Id: I148d98333051c399a4cb11bd9620b396f4eb261d
Reviewed-on: https://chromium-review.googlesource.com/456282
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43942}