VT: Deflake nothing captured view transition test.
Instead of relying on a double rAF, use waitForCompositorReady in
an attempt to deflake the test.
Also update the takeScreenshot condition to await the ready
promise instead of a double raf.
Bug: 378536381, 378637954
Change-Id: Icc54a556d7b26d9d74bb9b7fd39a25aef6adccd5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6041322
Commit-Queue: Vladimir Levin vmpstr@chromium.org
Reviewed-by: Kevin Ellis kevers@chromium.org
Cr-Commit-Position: refs/heads/main@{#1386391}