Improvements to Google Cloud Messaging for Chrome docs.
Added new left-side navigation in apps docs to show all messaging docs.
Added new paragraph on Google API client libraries.
Added Terms of Service doc and linked to it from service API reference.
Additional improvements to docs based on bug raised and
small re-ordering of list based on emal from Antony.
Note that I still need to find out what to do about error messages.
BUG=162175
Review URL: https://codereview.chromium.org/
11817002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@178067 0039d316-1c4b-4281-b951-d872f2087c98