<https://webkit.org/b/120013> Tighten up logic in HTMLTableRowsCollection

<https://webkit.org/b/120013> Tighten up logic in HTMLTableRowsCollection

Reviewed by Andy Estes.

I was looking for incorrect uses of hasLocalName in places where hasTagName should be used.
The use in HTMLTableRowsCollection looked like that kind of mistake, but when I tried to
make a test case to show the mistake, I found I could not. So I wrote assertions to restate
what I learned, and removed an unneeded null check and tightened up the code a bit. This
should make code size slightly smaller.

* html/HTMLTableRowsCollection.cpp:
(WebCore::assertRowIsInTable): Added. Asserts that the row's position in the table is consistent
with the invariants of how the collection class works. A row that is processed here would have
to be an immediate child of the table, or an immediate child of a table section that in turn is
an immediate child of the table.
(WebCore::isInSection): Added. Replaces the three more-specific helper functions. Unlike those,
this does not do a null check.
(WebCore::HTMLTableRowsCollection::rowAfter): Changed to use the two new functions.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@154288 268f45cc-cd09-0410-ab3c-d52691b4dbfc