Oilpan: Improve Oilpan support in EventContext and EventPath.

* TreeScopeEventContext
 Move to Oilpan heap.
* WindowEvnetContext
 Declare as STACK_ALLOCATED(), and remove Persistent<>s.
* EventPath
 Eliminate raw pointers.

BUG=

Review URL: https://codereview.chromium.org/336463002

git-svn-id: svn://svn.chromium.org/blink/trunk@176114 bbb929c8-8fbe-4397-9dbb-9b2b20218538