Add First and Last constant to WebInputEvent.

First part of two-sided patch to enable IPC enum checks.

BUG=246708

Review URL: https://codereview.chromium.org/165213002

git-svn-id: svn://svn.chromium.org/blink/trunk@167149 bbb929c8-8fbe-4397-9dbb-9b2b20218538