[DOM4] Have ProcessingInstruction inherit CharacterData

Have ProcessingInstruction inherit CharacterData as per the latest DOM4
specification:
http://dom.spec.whatwg.org/#processinginstruction

Previously, ProcessingInstruction was inheriting from Node as per DOM
Level 3.

Firefox already follows DOM4 but not IE10.
This change leads to less code duplication.

BUG=275465
R=haraken@chromium.org, jochen@chromium.org

Review URL: https://codereview.chromium.org/22911019

git-svn-id: svn://svn.chromium.org/blink/trunk@156390 bbb929c8-8fbe-4397-9dbb-9b2b20218538