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.

R=kevers@chromium.org

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}