Fix GN build with use_openssl=1

(This isn't quite sufficient as //third_party/openssl needs a BUILD.gn file
also. I'm doing that in https://codereview.chromium.org/183793004.)

BUG=none

Review URL: https://codereview.chromium.org/183833002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254019 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254019