Fix a bug that mousedown without mouseup in a frame disturbs click event in another frame.

If a mouse button is clicked but mouseup event for it is not dispatched
(it happens frequently because of context menus) and mousedown in
another frame is default-prevented, 'click' event for the latter mouse
click is not dispatched.

EventHandler::m_capturingMouseEventsNode wrongly continues to hold a node
for the first click, and it prevents hit-testing for the second click. We
should alwyas clear m_capturingMouseEventsNode in mousedown handling code.

BUG=262007

Review URL: https://chromiumcodereview.appspot.com/19735004

git-svn-id: svn://svn.chromium.org/blink/trunk@154698 bbb929c8-8fbe-4397-9dbb-9b2b20218538