Mouse event handlers use user gesture tokens on top-level LocalFrames.

The last mouseDown and mouseUp user gesture tokens are set on the top
level frame's EventHandler. For the FrameTree to be able to contain
RemoteFrames, we need to account for instances where the current
frame has RemoteFrame ancestors. This change causes the tokens to be
used on the EventHandler attached to the frame that is the root of the
local subtree.

This will not make gestures work with RemoteFrames, as that
will need a much larger refactoring. Eventually gesture
tracking will need to be synchronized across processes.

BUG=346764

Review URL: https://codereview.chromium.org/248083003

git-svn-id: svn://svn.chromium.org/blink/trunk@172633 bbb929c8-8fbe-4397-9dbb-9b2b20218538