[Shadow DOM] Add an assertion to make sure a visited node is not an active insertion...

[Shadow DOM] Add an assertion to make sure a visited node is not an active insertion point.
https://bugs.webkit.org/show_bug.cgi?id=86708

Reviewed by Eric Seidel.

Since InsertionPoint now has isActive() member function,
we can add an assertion to ComposedShadowTreeWalker::assertPrecondition()
using that function.

No new tests, no change in behavior.

* dom/ComposedShadowTreeWalker.h:
(WebCore::ComposedShadowTreeWalker::assertPrecondition):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@117534 268f45cc-cd09-0410-ab3c-d52691b4dbfc