Issue 6538, 6781 - Add code injection wrapper to snippets library

lib/content/snippets.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+/* eslint-env webextensions */
 /* eslint no-console: "off" */
 
 "use strict";
 
+function injectCode(code, dependencies = [])
+{
+  for (let dependency of dependencies)
+    code += dependency;
+
+  let script = document.createElement("script");
+
+  script.type = "application/javascript";
+  script.async = false;
+
+  // Firefox 58 only bypasses site CSPs when assigning to 'src',
+  // while Chrome 67 only bypasses site CSPs when using 'textContent'.
+  if (browser.runtime.getURL("").startsWith("chrome-extension://"))
+  {
+    script.textContent = code;
+    document.documentElement.appendChild(script);
+  }
+  else
+  {
+    let url = URL.createObjectURL(new Blob([code]));
+    script.src = url;
+    document.documentElement.appendChild(script);
+    URL.revokeObjectURL(url);
+  }
+
+  document.documentElement.removeChild(script);
+}
+
+function stringifyFunctionCall(func, ...params)
+{
+  return `"use strict";(${func})(${params.map(JSON.stringify).join(",")});`;
+}
+
+function makeInjector(injectable, ...dependencies)
+{
+  return (...args) => injectCode(stringifyFunctionCall(injectable, ...args),
+                                 dependencies);
+}
+
 function log(...args)
 {
   console.log(...args);
 }
 
 exports.log = log;