Make Math.random() deterministic in provisions

Replace Math.random() in sandbox with a seedable implementation so that
the script remains deterministic.

lib/sandbox.js

@@ -18,6 +18,8 @@
 
 const vm = require("vm");
 
+const seedrandom = require("seedrandom");
+
 const common = require("./common");
 const device = require("./device");
 const extensions = require("./extensions");
@@ -38,6 +40,8 @@ const sandbox = {
   declarations: null,
   extensionsCache: null,
   extensions: null,
+  clear: null,
+  rng: null,
   context: vm.createContext()
 }
 
@@ -54,6 +58,13 @@ class SandboxDate {
   }
 };
 
+function random() {
+  if (!sandbox.rng)
+    sandbox.rng = seedrandom(sandbox.timestamp);
+
+  return sandbox.rng();
+}
+
 
 class ParameterWrapper {
   constructor(path, attributes, unpacked, unpackedRevision) {
@@ -203,6 +214,11 @@ Object.defineProperty(sandbox.context, 'clear', {value: clear});
 Object.defineProperty(sandbox.context, 'commit', {value: commit});
 Object.defineProperty(sandbox.context, 'ext', {value: ext});
 
+// Monkey-patch Math.random() to make it deterministic
+sandbox.context.random = random;
+vm.runInContext("Math.random = random;", sandbox.context);
+delete sandbox.random;
+
 
 function errorToFault(err) {
   if (!err)
@@ -230,6 +246,7 @@ function run(script, globals, timestamp, deviceData, extensionsCache, startRevis
   sandbox.declarations = [];
   sandbox.extensions = {};
   sandbox.clear = null;
+  sandbox.rng = null;
 
   for (let n in sandbox.context)
     delete sandbox.context[n];