Clarify that Blink does not support a re-distribution across v0 and v1 shadow trees

I do not have a plan to support a re-distribution across v0 and v1 shadow trees.
It would be better to clarify it in the comment.

BUG=531990

Review URL: https://codereview.chromium.org/1877813002

Cr-Commit-Position: refs/heads/master@{#386606}
1 file changed