[GCM] Adding collapse key support to downstream messages

The initial API definition omitted collapse key support for downstream messages.

This patch adds the collapseKey property to the message parameter of
chrome.gcm.onMessage event, as well as collapse_key to gcm::GCMClient::IncomingMessage.

Tests on both API and GCMProfileService level were updated.

BUG=346855

Review URL: https://codereview.chromium.org/182483002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254235 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254235