Factor out the string lookup code from registry_controlled_domains

The string lookup code used by registry_controlled_domains is generally useful
as it provides for efficient lookup of strings in a set with no runtime
initialisation overhead.

Factor it out into a public LookupStringInFixedSet function and add unit tests.

BUG=348877
TEST=net_unittests

Review URL: https://codereview.chromium.org/1412343008

Cr-Commit-Position: refs/heads/master@{#359506}
10 files changed