Support default values for complex objects in mojom.

BUG=320082
R=darin@chromium.org

Review URL: https://codereview.chromium.org/139613005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248579 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248579
Mojo: More work towards sending message pipe handles over remote message pipes.

ProxyMessagePipeEndpoint::EnqueueMessage() needs to look at each handle
(i.e., dispatcher) and decide what to do.

This CL doesn't actually do anything yet exactly. I also need to check
(at a higher level) that we're not sending the peer handle over a given
handle, hence I'm splitting out this part now (since that work will
touch on the same things as this).

R=davemoore@chromium.org

Review URL: https://codereview.chromium.org/145483005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248580 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248580