Make TreeScopeEventContext have a RefPtr to TreeScope.rootNode to guard TreeScope.

      This fixes a use-after-free caused by TreeScope being freed while TreeScopeEventContext still needs it.
Because TreeScope itself isn't a RefCounted, guard it by having a RefPtr to treeScope.rootNode(), instead.

BUG=442806

Review URL: https://codereview.chromium.org/794123004

git-svn-id: svn://svn.chromium.org/blink/trunk@187435 bbb929c8-8fbe-4397-9dbb-9b2b20218538