Split EventContext.{cpp,h} into {Node,TouchEvent,TreeScope}EventContext.{cpp,h} files.

BUG=329445

Review URL: https://codereview.chromium.org/127783002

git-svn-id: svn://svn.chromium.org/blink/trunk@164747 bbb929c8-8fbe-4397-9dbb-9b2b20218538