Pepper: Fix FileIO host when no RenderProcessHost.

GetPluginAllowedToCallRequestOSFileHandle() doesn't properly handle the case
when a RenderProcessHost may no longer exist for a given render_process_id.

This change handles that case to prevent a crash observed by users.

BUG=348604

Review URL: https://codereview.chromium.org/197563002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256631 0039d316-1c4b-4281-b951-d872f2087c98
Mojo: Add an embedder-level multiprocess test of message pipe handle passing.

This is basically an end-to-end test.

R=yzshen@chromium.org

Review URL: https://codereview.chromium.org/197743002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256632 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256632