Set a display limit of 8 on multiple-item notifications items.

Set a display limit of 8 on multiple-item notifications items.

This limit is defined in the notification product requirement document,
section "Inbox Format for Multiple Messages". The limit only affects
the number of items displayed (to avoid notifications growing too tall).
The back end still handles any number of items. The maximum number is
declared in the public message_center_constants.h file so future code
can use it, for example to output a warning or return an error if
callers to the chrome.experimental.notification API ask for a
notification with more than the maximum number of displayable items.

BUG=161101

Review URL: https://chromiumcodereview.appspot.com/11638035

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@175132 0039d316-1c4b-4281-b951-d872f2087c98