arc: Optin dialog Update <style is="custom-style"> for Polymer 2

Wrapping <style is="custom-style"> with <custom-style> per Polymer 2
migration guide.

Bug: 931191
Test: Manual
Change-Id: I99330eecb273da43462cc834efb109b665af8a18
Reviewed-on: https://chromium-review.googlesource.com/c/1468785
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Long Cheng <lgcheng@google.com>
Cr-Commit-Position: refs/heads/master@{#631794}
1 file changed