Rename [ImplementedBy] => [PartialInterfaceImplementedAs]

Rename (separate CL for clarity) per comments:

Followup to:
IDL bindings: Switch 'implements' behavior to default to dispatch on |impl|
https://codereview.chromium.org/227163006/

TBR=haraken
BUG=360435

Review URL: https://codereview.chromium.org/225183011

git-svn-id: svn://svn.chromium.org/blink/trunk@170957 bbb929c8-8fbe-4397-9dbb-9b2b20218538