Bug 795542 - Part 1: Create Text(En|De)coderBase. r=bent a=tef+

--HG--
rename : dom/encoding/TextDecoder.h => dom/encoding/TextDecoderBase.h
rename : dom/encoding/TextEncoder.h => dom/encoding/TextEncoderBase.h
Bug 795542 - Part 2: Implement StringEncoding API objects in Workers. r=bent a=tef+