DevTools: copy protocol files to front-end to fix hosted mode

DevTools "hosted mode" is broken in a multiple ways:
- it uses sync XHR from inside the microtask (this was recentrly forbidden)
- as the protocol was split and moved around, the InspectorBackendHostedMode.js
  is not able to generate proper inspector backend commands

The patch fixes both problems. Here we:
- manually copy browser_protocol.json and js_protocol.json to the front-end
  folder so that hosted mode can rely on it
- preload protocol files into Runtime.cachedResources in hosted mode for future
  use of InspectorBackendHostedMode.js

BUG=623602
R=dgozman, pfeldman

Review-Url: https://codereview.chromium.org/2124203002
Cr-Commit-Position: refs/heads/master@{#404447}
6 files changed