Add managed policies for Native Messaging

This adds two new policies that will allow to block all or some native
messaging hosts.

BUG=237882

Review URL: https://codereview.chromium.org/118253005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243774 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243774
Introduce a hash_of_hashes dictionary.

It allows us to know whether the existing hashes dictionary can be trusted on startup (and thus whether seeding should be allowed).

Note: seeding a NULL preference is always allowed.

Review URL: https://codereview.chromium.org/126093007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243776 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243776
Moves CandidateWindow model to ui/base/ime.

BUG=325813
R=komatsu@chromium.org
TEST=build passes

Review URL: https://codereview.chromium.org/126363002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243777 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243777