insertOrderedList: Remove isListMergableContainer; Use isTable(Cell) instead

PTAL
It is better to name |isHTMLTableElement| as C++ does, it is defined in HTMLTableElement.h via macro.

I created spreadsheet which holds functions in htmlediting.cpp and CompositeCommand.h: https://docs.google.com/a/chromium.org/spreadsheets/d/1Yd9S1ezLuJFS5xbmr59lf2hz6WFm4oj843Mhj4EK1n4/edit?usp=sharing

Let's use these names if we implement corresponding function in JS.