Simplify content::InputEvent

With the removal of buffered input routing, content::InputEvent can be tailored
to suit current needs.  Remove the IPC::Message supporting InputEvent::Payload
construct, making InputEvent a lightweight wrapper of WebInputEvent and
LatencyInfo.

BUG=303219

Review URL: https://codereview.chromium.org/25690004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227589 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227589