css-shapes: Fix flakiness of shape-image-029.html
Wait until 'support/left-half-rectangle.png' loaded.
Bug: 372252514, 372756068
Change-Id: I106ab5e4de34be91f4365129011aec58f19c936a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5923354
Reviewed-by: Morten Stenshorne mstensho@chromium.org
Auto-Submit: Kent Tamura tkent@chromium.org
Commit-Queue: Kent Tamura tkent@chromium.org
Cr-Commit-Position: refs/heads/main@{#1367293}