Bug 840559 - Rename nsMediaError to MediaError. r=Ms2ger

--HG--
rename : content/html/content/src/nsMediaError.cpp => content/html/content/src/MediaError.cpp
rename : content/html/content/src/nsMediaError.h => content/html/content/src/MediaError.h
Bug 840559 - Convert MediaError to WebIDL. r=smaug