Assertion failure in WebCore::HTMLFrameElementBase::insertedIntoDocument()

Assertion failure in WebCore::HTMLFrameElementBase::insertedIntoDocument()
https://bugs.webkit.org/show_bug.cgi?id=50312

Reviewed by Alexey Proskuryakov.

Source/WebCore:

Removed the ASSERT and updated the comment.

Test: fast/frames/assert-on-insertedIntoDocument.html

* html/HTMLFrameElementBase.cpp:
(WebCore::HTMLFrameElementBase::insertedIntoDocument):

LayoutTests:

* fast/frames/assert-on-insertedIntoDocument-expected.txt: Added.
* fast/frames/assert-on-insertedIntoDocument.html: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105463 268f45cc-cd09-0410-ab3c-d52691b4dbfc