GitLab

As the first step of implementing Idle Time Spell Checker, this patch adds
a new runtime enabled feature IdleTimeSpellChecking, and exits the existing
spell-checking code paths when the feature is enabled.

BUG=671922

Review-Url: https://codereview.chromium.org/2566043003
Cr-Commit-Position: refs/heads/master@{#437840}