Allow idle notification when incremental marking is turned off.
Allow idle notification when incremental marking is turned off.

BUG=
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/725583005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#25346}

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