[IMP] devtools: add support for file urls on chrome

This commit ensures that the hook will properly be loaded on file urls
when using chrome (this was already working in firefox). It is still
necessary to check the "Allow access to file URLs" option in the
extensions manager for this to work properly.

tools/devtools/manifest-chrome.json

@@ -14,7 +14,7 @@
     "default_popup": "popup_app/popup.html"
   },
   "permissions": ["scripting", "storage"],
-  "host_permissions": ["http://*/*", "https://*/*"],
+  "host_permissions": ["http://*/*", "https://*/*", "file://*"],
   "content_security_policy": {
     "script-src": "self",
     "object-src": "self"