Factor out SameHost() in registry_controlled_domains

Factor out SameHost() in registry_controlled_domains

This allows clients to replicate SameDomainOrHost functionality in a more 
flexible and efficient way. One example use case is when there is a fixed
|gurl_0|, and many different |gurl_i|'s need to be checked against |gurl_0|.

After this CL, an efficient solution will be to save |gurl_0| and 
|domain_and_registry_0| = GetDomainAndRegistry(gurl_0), and then for for every
incoming |gurl_i|, calculate GetDomainAndRegistry(gurl_i), and fall back to
SameHost(gurl_0, gurl_i) not both of them have both domains.

BUG=

[pkalinnikov, 2016-10-05 14:13:54]

Description was changed from

==========
Factor SameHost check out.

BUG=
==========

to

==========
Factor the SameHost check out.

This is needed to allow clients to "repeat" SameDomainOrHost functionality in a
more flexible and efficient way.

One example use case is as follows:
 - There is a fixed |gurl1|.
 - Many different |gurl2|'s come for being checked against |gurl1|.
Solution:
 - Save |gurl1| and |domain_and_registry| = GetDomainAndRegistry(gurl1).
 - For every incoming |gurl2| GetDomainAndRegistry(gurl2), and "repeat"
SameDomainOrHost after the point where we have both domains.

BUG=
==========

[pkalinnikov, 2016-10-05 14:14:45]

Description was changed from

==========
Factor the SameHost check out.

This is needed to allow clients to "repeat" SameDomainOrHost functionality in a
more flexible and efficient way.

One example use case is as follows:
 - There is a fixed |gurl1|.
 - Many different |gurl2|'s come for being checked against |gurl1|.
Solution:
 - Save |gurl1| and |domain_and_registry| = GetDomainAndRegistry(gurl1).
 - For every incoming |gurl2| GetDomainAndRegistry(gurl2), and "repeat"
SameDomainOrHost after the point where we have both domains.

BUG=
==========

to

==========
Factor the SameHost check out.

This is needed to allow clients to "repeat" SameDomainOrHost functionality in a
more flexible and efficient way.

One example use case is as follows:
 - There is a fixed |gurl1|.
 - Many different |gurl2|'s come for being checked against |gurl1|.
Solution:
 - Save |gurl1| and |domain_and_registry| = GetDomainAndRegistry(gurl1).
 - For every incoming |gurl2| GetDomainAndRegistry(gurl2), and "repeat"
   SameDomainOrHost after the point where we have both domains.

BUG=
==========

[pkalinnikov, 2016-10-05 14:16:57]

pkalinnikov@chromium.org changed reviewers:
+ engedy@chromium.org

[pkalinnikov, 2016-10-05 14:16:57]

Balazs, PTAL and verify the approach. This is for supporting the CL 2288023003.

[pkalinnikov, 2016-10-05 14:17:20]

The CQ bit was checked by pkalinnikov@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-10-05 14:17:38]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-10-05 15:53:04]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-10-05 15:53:06]

Dry run: This issue passed the CQ dry run.

[engedy, 2016-10-21 08:42:19]

Description was changed from

==========
Factor the SameHost check out.

This is needed to allow clients to "repeat" SameDomainOrHost functionality in a
more flexible and efficient way.

One example use case is as follows:
 - There is a fixed |gurl1|.
 - Many different |gurl2|'s come for being checked against |gurl1|.
Solution:
 - Save |gurl1| and |domain_and_registry| = GetDomainAndRegistry(gurl1).
 - For every incoming |gurl2| GetDomainAndRegistry(gurl2), and "repeat"
   SameDomainOrHost after the point where we have both domains.

BUG=
==========

to

==========
Factor out SameHost() in registry_controlled_domains

This allows clients to replicate SameDomainOrHost functionality in a more 
flexible and efficient way. One example use case is when there is a fixed
|gurl_0|, and many different |gurl_i|'s need to be checked against |gurl_0|.

After this CL, an efficient solution will be to save |gurl_0| and 
|domain_and_registry_0| = GetDomainAndRegistry(gurl_0), and then for for every
incoming |gurl_i|, calculate GetDomainAndRegistry(gurl_i), and fall back to
SameHost(gurl_0, gurl_i) not both of them have both domains.

BUG=
==========

[engedy, 2016-10-21 08:43:22]

LGTM % nit, but I'd defer to Peter.

https://codereview.chromium.org/2393163002/diff/1/net/base/registry_controlle...
File net/base/registry_controlled_domains/registry_controlled_domain.h (right):

https://codereview.chromium.org/2393163002/diff/1/net/base/registry_controlle...
net/base/registry_controlled_domains/registry_controlled_domain.h:198: //
Returns whether the hosts of the GURLs are identical.
nit: Maybe we could put this into the anonymous namespace as well, and only move
the declaration to the header once we actually start using it?

When we do that, we should clarify in one more line what identical means in this
context.