Nested view transition: runtime flag & parsing
This adds the initial runtime flag & property parsing,
as specified in the CSSWG resolution:
w3c/csswg-drafts#10334 (comment)
This only deals with parsing as a custom ident, will add
the keywords, rendering etc. in subsequent CLs.
ChromeStatus: https://chromestatus.com/feature/5162799714795520
I2P:
https://groups.google.com/a/chromium.org/g/blink-dev/c/iG4WDZOOzxY
Bug: 347947051
Change-Id: I102c2d4e35efd305a6d43af7bfbbeb81b68a8393
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5741509
Reviewed-by: Khushal Sagar khushalsagar@chromium.org
Commit-Queue: Noam Rosenthal nrosenthal@chromium.org
Cr-Commit-Position: refs/heads/main@{#1336513}