Fix uncaught ReferenceError that prevents IN_BROWSER from being initialized

!2 broke this code in browsers: tails#18616.

In a browser, evaluating "if(process)" raises a ReferenceError exception.
As a result, the following variables initialization are not evaluated.
In particular, IN_BROWSER is not set, so later on we would run testMirrorNode(),
which is itself broken in a browser, instead of testMirrorBrowser().