HID backend.

This is a continuation of https://codereview.chromium.org/143883005.
[ps#1 here is (post-revert) ps#12 there.]

BUG=290428
TBR=miket@chromium.org

Review URL: https://codereview.chromium.org/150773004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248250 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248250