Chrome Version       : 51.0.2704.106 (Developer Build) (64-bit)
Other browsers tested:
  Firefox 4.x: OK

What steps will reproduce the problem?

1. Load extension.
2. Observe unexpected alert that says, "typeof msgs[k] is undefined?!"
3. If the alert does not appear, try reloading the extension a few times.

What is the expected result?

No alert message.

What happens instead of that?

An alert message sometimes, but not always.

Unfortunately, reducing a test case is non-trivial because the behavior is intermittent.  However, the attached extension on my machine shows the unexpected alert 10 out of the last 11 times I loaded it.  The relevant code that shows unexpected behavior on my machine is:

    if (msgs[k]) { alert("typeof msgs[k] is " + typeof msgs[k] + "?!"); }

I don't think msgs[k] should be truthy and undefined.
 

Deleted: chromium-race-extension.zip 1.0 KB

chromium-race-extension.zip 1.0 KB Download