Initial implementation of Opaque Response Blocking (ORB).

This CL implements most of the Opaque Response Blocking (ORB) algorithm
from https://github.com/annevk/orb.  Major exceptions:

1) One of last steps in ORB asks to sniff the whole response body to
   confirm whether the response contains valid Javascript.  ORB's
   algorithm results in the most correct and secure behavior, but
   requires more work to implement.  For now Chromium's implementation
   determines `is_surely_not_javascript` by sniffing the first 1024
   bytes with CORB's confirmation sniffers (see
   IsSensitiveHtmlXmlOrJson).  This approximates ORB's behavior and
   improves security (e.g. applying protection to HTML and XML served as
   application/octet-stream), although unlike full ORB the current
   implementation fails open rather than closed.

2) Chromium continues blocking responses by injecting an empty response
   body and stripping HTTP headers (rather than by injecting a network
   error).

3) Other differences are tracked in the "ORB v0.1 vs full ORB" section
   of the doc here:
   https://docs.google.com/document/d/1qUbE2ySi6av3arUEw5DNdFJIKKBbWGRGsXz_ew3S7HQ/edit#heading=h.mptmm5bpjtdn

There are also some other minor differences that hopefully can be
removed over time.

Bug: 1178928
Change-Id: I4f018bdb75b5ff7d10233271e42476a9cc50bf2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3203018


Reviewed-by: Charles Reis <creis@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/main@{#963088}