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}

chromeos/components/camera_app_ui/resources/src/css/main.css

@@ -481,6 +481,11 @@ body.video.recording-ui-paused #pause-recordvideo .two-bars {
   --bar-size: 0px;
 }
 
+body.video.taking #recordvideo {
+  /* Prevent white background shown outside red dot area (crbug.com/1125896) */
+  background: transparent;
+}
+
 body:not(.video) #toggle-mic,
 body:not(.multi-camera) #switch-device {
   visibility: hidden;