GitLab

Make two changes to how we initialize features for contexts in extension
native bindings:
- Treat runtime specially. This is necessary because certain runtime
  methods should be exposed only if there exists an extension that can
  connect to the given context (e.g., an extension that cooperates with
  a given website).  Simply checking the fature availability is
  insufficient. Update unittests to match.
- Treat web contexts specially.  Web contexts only ever have a handful
  (3) APIs that are potentially available to them, and are by far the
  most commonly created context. As such, add a fast path for them by
  only checking those possible features. (This matches logic in the
  JS bindings system).

Bug: 653596
Change-Id: I57f50b6f762f578679771c5e07a1b3ce31da3e0c
Reviewed-on: https://chromium-review.googlesource.com/571962Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486965}