fix(ses-demo): adapt to disabled Date.now() throwing (#2357)

Closes: #XXXX
Refs: #910 (comment)
#1718
#2354
#910

## Description

While investigating #2354 , I just
tried it locally by visiting

    file:///.../endojs/endo/packages/ses/demos/console/index.html

and

    file:///.../endojs/endo/packages/ses/demos/challenge/index.html

in my browser. The first is the SES demo console, which worked just
fine. The second is the SES Escape Room, which still relied on the
disabled `Date.now()` not throwing. Indeed, before
#1718 a disabled (secure mode)
`Date.now()` returned `NaN`. But
#1718 changed it to throw.

What's strange is that in #1718 I
revise endojs/endo/packages/ses/demos/challenge/index.html to adjust the
text to say that `Date.now()` is "disabled" rather than "NaN". But I
didn't fix the escape room code.

Although I found this while investigating
#2354 , this is a completely
distinct bug that is unrelated to
#2354 . This PR itself does nothing
to fix #2354 .

### Security Considerations

none
### Scaling Considerations

none
### Documentation Considerations

All our docs that link to an explain the Escape Room challenge need to
be revisited, especially once #2354
is fixed. For example,
https://agoric.com/blog/technology/a-taxonomy-of-security-issues , which
I just verified links to the broken page reported at
#2354

### Testing Considerations

It is frustrating that the Escape Room is broken at least since
#1718 , and also broken by
#2354 for an undetermined period of
time, without anyone noticing until now. It would be good to bring that
site under some kind of automated testing.

### Compatibility Considerations

none
### Upgrade Considerations

none

Author: erights

packages/ses/demos/challenge/main.js

@@ -250,6 +250,18 @@ lockdown();
       return into.slice(0, offset) + char + into.slice(offset + 1, into.length);
     }
 
+    function dateNow() {
+      try {
+        return Date.now();
+      } catch (err) {
+        if (err instanceof TypeError) {
+          // assume we cannot measure time because of err
+          return NaN;
+        }
+        throw err;
+      }
+    }
+
     /**
      * @param {string} base
      * @param {number} offset
@@ -273,9 +285,9 @@ lockdown();
         const delays = new Map();
         for (let c = 0; c < 36; c++) {
           const guessedCode = buildCode(base, offset, c);
-          const start = Date.now();
+          const start = dateNow();
           guess(guessedCode);
-          const elapsed = Date.now() - start;
+          const elapsed = dateNow() - start;
           delays.set(toChar(c), elapsed);
           yield; // allow UI to refresh
           // if our guess was right, then on the last character