Label IPv6 reachability checks in NetLog

Chrome's internal DNS resolver checks whether the host currently has
global IPv6 connectivity by creating a socket connected to Google Public
DNS' IPv6 address.  These show up in the NetLog as UDP_SOCKET objects
with a source dependency of the HOST_RESOLVER_IMPL_REQUEST.

This change adds an explicit IPV6_REACHABILITY_CHECK source type that
will show up instead, to clarify the purpose of the UDP sockets.

(Note that no packets are actually sent as a result of the checks; UDP
doesn't need to send any packets to connect, and the HostResolverImpl
doesn't send any data over the socket explicitly.)

BUG=331746

Review URL: https://codereview.chromium.org/141023002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245336 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245336
Allow inactive windows to be created with Linux Aura.

BUG=325142

Review URL: https://codereview.chromium.org/100623008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245338 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245338
Mojo: Move system embedder API files to mojo/system/embedder.

(More changes to come -- in particular to reduce exports from
mojo_system_impl -- but one thing at a time....)

R=sky@chromium.org

Review URL: https://codereview.chromium.org/139823003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245339 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245339