Bug 1368068 - Check if sandbox' window is dead before evaluating script; r=ato

If the previous sandbox has a dead WindowProxy reference, we should blow
away the sandbox.

This can be reproduced when a sandbox persists across multiple invocations
of the Execute Script or Execute Async Script commands, but a window is
closed in between.  This will normally produce a TypeError: can't access
dead object error.

Signed-off-by: Andreas Tolfsen <ato@sny.no>
MozReview-Commit-ID: 2xdSnKV4YHI

testing/marionette/evaluate.js

@@ -299,6 +299,31 @@ evaluate.toJSON = function(obj, seenEls) {
   return rv;
 };
 
+/**
+ * Cu.isDeadWrapper does not return true for a dead sandbox that was
+ * assosciated with and extension popup. This provides a way to still
+ * test for a dead object.
+ *
+ * @param {?} obj
+ *     A potentially dead object.
+ * @param {string} prop
+ *     Name of a property on the object.
+ *
+ * @returns {boolean}
+ *     True if |obj| is dead, false otherwise.
+ */
+evaluate.isDead = function(obj, prop) {
+  try {
+    obj[prop];
+  } catch (e) {
+    if (e.message.includes("dead object")) {
+      return true;
+    }
+    throw e;
+  }
+  return false;
+};
+
 this.sandbox = {};
 
 /**
@@ -434,7 +459,7 @@ this.Sandboxes = class {
   get(name = "default", fresh = false) {
     let sb = this.boxes_.get(name);
     if (sb) {
-      if (fresh || sb.window != this.window_) {
+      if (fresh || evaluate.isDead(sb, "window") || sb.window != this.window_) {
         this.boxes_.delete(name);
         return this.get(name, false);
       }