CrOS File Manager: Fix volume slider UI bug in quick view.

This CL fix the issue where volume slider is hidden in CrOS file manager
quick view. This CL also removes pointer CSS as it was implemented in
native controls.

Bug: 945799
Change-Id: I944b6fc921e55d82c9b016a9338420b7f9a17fcf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462050
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815542}

ui/file_manager/file_manager/foreground/elements/files_safe_video_webview_content.css

@@ -30,21 +30,15 @@ img {
   display: none;
 }
 
-video::-webkit-media-controls-timeline,
-video::-webkit-media-controls-mute-button,
-video::-webkit-media-controls-play-button {
-  cursor: pointer;
-}
-
-video::-webkit-media-controls-fullscreen-button,
-video::-webkit-media-controls-volume-slider {
+video::-webkit-media-controls-fullscreen-button {
   display: none;
 }
 
 video:focus,
 video::-webkit-media-controls-timeline:focus,
 video::-webkit-media-controls-mute-button:focus,
-video::-webkit-media-controls-play-button:focus {
+video::-webkit-media-controls-play-button:focus,
+video::-webkit-media-controls-volume-slider:focus {
   outline: 2px solid rgba(66, 133, 244, 0.5);
   outline-offset: -2px;
 }