Deflake http/tests/local/style-access-before-stylesheet-loaded.html

The test has two issues:

1. The test code is supposed to be run while the style sheet is loading,
but fails to do so because a pending style sheet blocks the evaluation
of any subsequent script elements. This patch fixes it by triggering the
test code with a 'DOMContentLoaded' event listener.

2. The test case verifies that `offsetLeft` should be fast by measuring
the total runtime of 1000 calls, and comparing that to 500ms. This can
fail on debug builds. This patch changes it into asserting that no style
recalc is done, since this is what the original bug [1] is about.

[1] https://bugs.webkit.org/show_bug.cgi?id=14742

Fixed: 1356728
Change-Id: I692a799bb0e0d2ffc3e6c5d7d2b4b2cfb48907dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3857529
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Auto-Submit: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1039870}