Revert "webvtt: Add background opacity in mac"

This reverts commit 8e606757.

Reason for revert: Caused crbug.com/964244

Original change's description:
> webvtt: Add background opacity in mac
>
> To test, open a video that has a WebVTT captions file. For example, https://www.iandevlin.com/html5test/webvtt/html5-video-webvtt-sample.html is a nice example.
>
> Introduce new background opacity text track property for webvtt captions and wire it to apply opacity to the WebVTT caption HTML element.
>
> Bug: 897730
> Change-Id: Ic8dabc2279117b7ff12eb75a9daa8d26c9d9bb25
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1592622
> Commit-Queue: Abigail Klein <abigailbklein@google.com>
> Auto-Submit: Abigail Klein <abigailbklein@google.com>
> Reviewed-by: Avi Drissman <avi@chromium.org>
> Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#659116}

Bug: 964244
Change-Id: I76ce88cee600ed98d48426b49d1d88a40d95e507
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1622433
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Auto-Submit: Abigail Klein <abigailbklein@google.com>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662163}

ui/native_theme/caption_style_mac.mm

@@ -47,14 +47,12 @@ std::string GetMAForegroundColorAndOpacityAsCSSColor() {
   return color_utils::SkColorToRgbaString(rgba_color);
 }
 
-std::string GetMABackgroundColorAndOpacityAsCSSColor() {
+std::string GetMABackgroundColorAsCSSColor() {
   base::ScopedCFTypeRef<CGColorRef> cg_color(
       MACaptionAppearanceCopyBackgroundColor(kUserDomain, nullptr));
-  float opacity = MACaptionAppearanceGetBackgroundOpacity(kUserDomain, nullptr);
 
-  SkColor rgba_color =
-      SkColorSetA(skia::CGColorRefToSkColor(cg_color.get()), 0xff * opacity);
-  return color_utils::SkColorToRgbaString(rgba_color);
+  return color_utils::SkColorToRgbaString(
+      skia::CGColorRefToSkColor(cg_color.get()));
 }
 
 // The MA text scale is a float between 0.0 and 2.0; this function converts it
@@ -127,7 +125,7 @@ CaptionStyle CaptionStyle::FromSystemSettings() {
   CaptionStyle style;
 
   style.text_color = GetMAForegroundColorAndOpacityAsCSSColor();
-  style.background_color = GetMABackgroundColorAndOpacityAsCSSColor();
+  style.background_color = GetMABackgroundColorAsCSSColor();
   style.text_size = GetMATextScaleAsCSSPercent();
   style.text_shadow = GetMATextEdgeStyleAsCSSShadow();