Make sure that Content Handled application can be connected multiple times.

This CL updates ApplicationManager so that it always reconnect to an
application that is already launched.

To obtain this results, the following changes habe also been made:

1) The Loader interface has been simplified:
  - The pipe to the shell if given during the Load request, so that loader do
    not need to called RegisterLoadedApplication
2) PngViewer and PdfViewer allows multiple embedder to embed those.
3) For the second connection forward, Sky issue network request to get the
   content of the page to display.

R=aa@chromium.org

Review URL: https://codereview.chromium.org/741453002