mac: Don't set background color from a frame until frame is displayed
The first frame for dark-themed new tab pages still has a background
color of white, even though the frame's content is all black tiles.
Before this patch, we would set the background color to white, and then
later put up the black tiles in front, resulting in a white flash.
This updates all of those parts atomically in the same CATransaction.
BUG=470669
Review-Url: https://codereview.chromium.org/2866983002
Cr-Commit-Position: refs/heads/master@{#469919}
2 files changed
