GitLab

Previously extensions::Dispatcher was responsible for creating ScriptContexts
then Adding them to its ScriptContextSet. Now, ScriptContextSet creates them
itself. This prepares the Extension system for later changing to lazily or
asynchronously creating ScriptContexts.

BUG=466373
R=rockot@chromium.org

Review URL: https://codereview.chromium.org/995283004

Cr-Commit-Position: refs/heads/master@{#320971}