DevTools: implement proxy server for hosted mode

Completed:
- Simple web server for serving static assets using node
- Proxies requests for certain files
- In-memory cache of proxied files from previous requests
- Add npm script "npm run server" to start the web server
- Users can use a custom port: "PORT=3000 npm run server"

Notes:
- Sends dummy response for InspectorBackendCommands.js because it’s not needed
for hosted mode and avoids a console error

BUG=629914

Review-Url: https://codereview.chromium.org/2167413002
Cr-Commit-Position: refs/heads/master@{#408018}
5 files changed