If enabled, SafeBrowsing URL checks will behave in the same way as when Network
Service is enabled. In other words, they don't defer starting requests or
following redirects, no matter on desktop or mobile. Instead they only defer
response processing.

Currently the feature is disabled by default. Follow-up work:
- Make the corresponding change on Android WebView.
- Following the finch experiment process, send out a doc/email to request a
  finch experiment.
- Remove the old behavior eventually.

Bug: 715673
Change-Id: I9b23efc58965a82dcb3ba0ccaa91a77bdccbca92
Reviewed-on: https://chromium-review.googlesource.com/625034


Commit-Queue: Yuzhu Shen <yzshen@chromium.org>
Reviewed-by: Varun Khaneja (slow) <vakh@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496481}