Bug 838559 - Rename nsHTMLMenuElement to HTMLMenuElement. r=Ms2ger

--HG--
rename : content/html/content/src/nsHTMLMenuElement.cpp => content/html/content/src/HTMLMenuElement.cpp
rename : content/html/content/src/nsHTMLMenuElement.h => content/html/content/src/HTMLMenuElement.h
Bug 838559 - Convert HTMLMenuElement to WebIDL. r=Ms2ger, r=peterv