Only add extensions to service worker schemes if the feature is on
Only add extensions to service worker schemes if the feature is on

Using the chrome-extensions:// scheme with service workers is still
experimental, but there's a bug in the M46 branch where some code to
support this is causing a regression. This feature should really only be
enabled in trunk right now, so this patch changes things to turn it on
only when the extension behavior features system says it should be on
(this is controlled in the _behavior_features.json file, which currently
allows it on trunk but no other channels).

BUG=525353

Review URL: https://codereview.chromium.org/1384793002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#352364}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk