Prepare ResourceFetcher to not loading main resource

- Trigger static locals on main frame. We used to do that
  as a part of loading main resource, but now we don't.
- Allow null Resource in DispatchDidReceiveResponse, it's
  not actually needed and will be missing for main resources.

Bug: 855189
Change-Id: I51d2a433501bfdff4dba677d61210b62bec58802
Reviewed-on: https://chromium-review.googlesource.com/c/1401674
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622045}
3 files changed