The return type of {CallFPImpl} differs between declaration and
definition (int32_t vs intptr_t). Even though our bots seem to be fine
with this, I got a compile error on mac.

R=mstarzinger@chromium.org

Change-Id: I4fb26cbe712c090d08dfcd2614a7af78de09e448
Reviewed-on: https://chromium-review.googlesource.com/909428Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51196}