Fix bugs in Native Messaging documentation

1. Path was not excaped properly in the example manifest file.
2. Docs didn't specify how a native messaging host can respond to a message
   sent using sendNativeMessage().

BUG=312884, 311266
R=mpcomplete@chromium.org

Review URL: https://codereview.chromium.org/79083003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236636 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236636