remove some isolate::currents from api.cc
remove some isolate::currents from api.cc

R=svenpanne@chromium.org

BUG=

Review URL: https://codereview.chromium.org/691513005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#25471}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh