blink/transitions: Make WA-API work for shared element transitions.

Fix the pseudo argument parsing for ::transition* pseudo elements in the
WA-API.

R=flackr@chromium.org, vmpstr@chromium.org

Change-Id: I746ce247b95f895ce41d11e9e45d5ce92e6ea864
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3513607
Auto-Submit: Khushal Sagar <khushalsagar@chromium.org>
Reviewed-by: Vladimir Levin <vmpstr@chromium.org>
Commit-Queue: Khushal Sagar <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#979789}