Hook up the password screen for the out of process PDF plugin.

This hooks up the password screen which is shown if the document is password protected.
It responds to requests to passwords for the plugin and responds with the password
text if it is accepted.

BUG=303491
R=arv@chromium.org, koz@chromium.org

Review URL: https://codereview.chromium.org/165973003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253034 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253034