CCA: Make record button background transparent in taking

Set record button background to transparent in taking state for
preventing it shown outside red dot area.

Bug: 1125896
Test: Manually test record functionality on dragonair.
Change-Id: Ib9260a5df1c7f47b82cb8f47dcbf8ea333ebc121
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2398029


Commit-Queue: Inker Kuo <inker@chromium.org>
Reviewed-by: Shik Chen <shik@chromium.org>
Auto-Submit: Inker Kuo <inker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805625}