Implement GetSelectedText() for the OOP PDF plugin.

This change exposes a SetSelectedText() function to the plugin so it can
eagerly notify the plugin host what the currently selected text is. This allows
the call to GetSelectedText() to return synchronously with the last selected
text, rather than the empty string.

R=joi@chromium.org, piman@chromium.org, raymes@chromium.org

Review URL: https://codereview.chromium.org/127343003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244827 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244827