Mousedown on datalist indicator should not open autofill popup.

We don't want to notify mousedown events on datalist indicator to an event
listener registered by autofill code because click event on the datalist
indicator will open datalist popup.

We override Node::preDispatchEventHandler to stop propagation. Node::
defaultEventHandler is too late to do it because it is called after DOM
event listeners.

BUG=331734

Review URL: https://codereview.chromium.org/124953003

git-svn-id: svn://svn.chromium.org/blink/trunk@164505 bbb929c8-8fbe-4397-9dbb-9b2b20218538