Changes to deletion/ownership of nodes in the client lib.

Nodes are now owned by the view manager. Constructors/destructors moved to private/protected. The ViewManager now maintains a map of id->node.
Adds an observer method for destruction. Clients will need to implement this to invalidate their pointer (perhaps I should invent a node smart ptr).
Adds lib tests for node removal, destruction, and connection destruction (when a connection is destroyed, all nodes it created should be destroyed).
Adds a client notification from the service to notify other clients of node destruction & some tests.

R=sky@chromium.org
http://crbug.com/365012

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=269414

Review URL: https://codereview.chromium.org/274733004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269426 0039d316-1c4b-4281-b951-d872f2087c98

• mojo/examples/sample_view_manager_app/sample_view_manager_app.cc[diff]
• mojo/services/public/cpp/view_manager/lib/view_manager.cc[diff]
• mojo/services/public/cpp/view_manager/lib/view_manager_private.cc[diff]
• mojo/services/public/cpp/view_manager/lib/view_manager_private.h[diff]
• mojo/services/public/cpp/view_manager/lib/view_manager_synchronizer.cc[diff]
• mojo/services/public/cpp/view_manager/lib/view_manager_synchronizer.h[diff]
• mojo/services/public/cpp/view_manager/lib/view_tree_node.cc[diff]
• mojo/services/public/cpp/view_manager/lib/view_tree_node_private.cc[diff]
• mojo/services/public/cpp/view_manager/lib/view_tree_node_private.h[diff]
• mojo/services/public/cpp/view_manager/tests/view_manager_unittest.cc[diff]
• mojo/services/public/cpp/view_manager/tests/view_tree_node_unittest.cc[diff]
• mojo/services/public/cpp/view_manager/view_manager.h[diff]
• mojo/services/public/cpp/view_manager/view_tree_node.h[diff]
• mojo/services/public/cpp/view_manager/view_tree_node_observer.h[diff]
• mojo/services/public/interfaces/view_manager/view_manager.mojom[diff]
• mojo/services/view_manager/root_node_manager.cc[diff]
• mojo/services/view_manager/root_node_manager.h[diff]
• mojo/services/view_manager/view_manager_connection.cc[diff]
• mojo/services/view_manager/view_manager_connection.h[diff]
• mojo/services/view_manager/view_manager_connection_unittest.cc[diff]