Address issues with parse-input-arguments-018.https.html

The test:

 external/wpt/css/css-paint-api/parse-input-arguments-018.https.html

is flakily failing or crashing. This turns out to have multiple causes:

 1) The generated random string that is part of inputArguments may
    not be a valid identifier and thus some invocations of
    registerPaint() fail before the input arguments are compared.

 2) Because of the thread setup used when running the test, the
    assumption about when a definition can be null does not hold.

To address (1), adjust the test to generate strings that are valid
identifiers. For (2), stay consistent with the associated
PaintWorklet's configuration by checking its `is_paint_off_thread_`
flag.

Fixed: 40786499
Change-Id: I3bd1d5eac067e7b9fbc201cb32c69c0a946baab5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5401738


Auto-Submit: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1281898}