Make event.path return every nodes in the event path, instead of erasing nodes on descendant trees.

The spec side issue is: https://www.w3.org/Bugs/Public/show_bug.cgi?id=25423

BUG=378143

Review URL: https://codereview.chromium.org/302453010

git-svn-id: svn://svn.chromium.org/blink/trunk@175034 bbb929c8-8fbe-4397-9dbb-9b2b20218538