Update deprecation message for multiple shadow roots

Multiple shadow roots will be removed in M63.

Bug: 489947
Change-Id: I537786957435cd491454dc8e97de3341420c274a
Reviewed-on: https://chromium-review.googlesource.com/640432
Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Takayoshi Kochi <kochi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#498064}
51 files changed