This uses a WebAssembly runtime stub for the out-of-line {DoubleToI}
computation instead of a builtin specific to a certain Isolate. It is
another step towards making {WasmCode} independent of the Isolate.

R=clemensh@chromium.org
BUG=v8:7424

Change-Id: Ifdaa8e2511b67468d75e0e4aaf70010ced04a11a
Reviewed-on: https://chromium-review.googlesource.com/1098672
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53702}