DevTools: Extract and lazy-load the "extensions" module

The module is only loaded once the first extension is registered.
DevToolsExtensionAPI.js contents have been merged into ExtensionAPI.js

R=caseq, pfeldman, vsevik
NOTRY=true

Review URL: https://codereview.chromium.org/167073002

git-svn-id: svn://svn.chromium.org/blink/trunk@167286 bbb929c8-8fbe-4397-9dbb-9b2b20218538