GitLab

This allows ime extensions creat window with proper shadow through css and a round button as in this mock:
https://folio.googleplex.com/chromeinput/final/voice#%2F00%20VK_start%20to%20speak.png

BUG=425539

Review URL: https://codereview.chromium.org/643283002

Cr-Commit-Position: refs/heads/master@{#300697}