Insert dummy content script at every location.

This is needed for propagating the extension's permissions
to the extension's iframe, in the rare event that the PDF is
loaded in a sub frame, and the extension does not have access to the
top frame. For instance, when a http:-PDF file is embedded in a
local file, while "Allow access to local URLs" is disabled.

Note: Propagating permissions by inserting content scripts is an
 undocumented feature (http://crbug.com/302548).

Whenever it breaks, the issue (cross-domain permissions for XHR)
can be solved by using a content script that gets the blob using
the XMLHttpRequest API, followed by `postMessage` (via transferables)
to efficiently pass the arraybuffer back to the PDF Viewer.

extensions/chromium/manifest.json

@@ -22,7 +22,12 @@
    */
   "key": "MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC4zyYTii0VTKI7W2U6fDeAvs3YCVZeAt7C62IC64IDCMHvWy7SKMpOPjfg5v1PgYkFm+fGsCsVLN8NaF7fzYMVtjLc5bqhqPAi56Qidrqh1HxPAAYhwFQd5BVGhZmh1fySHXFPE8VI2tIHwRrASOtx67jbSEk4nBAcJz6n+eGq8QIDAQAB",
   "content_scripts": [{
-    "matches": ["file://*/*"],
+    "matches": [
+      "http://*/*",
+      "https://*/*",
+      "ftp://*/*",
+      "file://*/*"
+    ],
     "js": ["nothing.js"]
   }],
   "mime_types": [