Update focus-navigation WPTs to ShiftKey for backward navigation

We can now use test_driver.action_sequence() and keyDown/keyUp to
send shift-tab for focus-navigation. The relevant WPT
focus-navigation tests can be removed from TestExpectations.

This patch also fix a bug in focus-utils.js, where we were using
`E050` for shift key when we should be using `E008` [1].

[1] https://source.chromium.org/chromium/chromium/src/+/main:chrome/test/chromedriver/key_converter.cc;l=133;drc=fe132eeb21687c455d695d6af346f15454828d01

Change-Id: Ia8a330b026265badf4ec9107705b9486a01edf73
Fixed: 40542192
Bug: 40597271
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5463903


Reviewed-by: Joey Arhar <jarhar@chromium.org>
Auto-Submit: Di Zhang <dizhangg@chromium.org>
Commit-Queue: Di Zhang <dizhangg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1289489}