Windows Captions Settings: do not use white as a default colour.

Default colour was white most likely by mistake which forced white as
the default background and foreground colour for captions.

Bug: 1000609
Change-Id: I87614e191ec3b3a1424f371d208691a7b6effa32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786250Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693951}

ui/native_theme/caption_style_win.cc

@@ -69,7 +69,7 @@ void GetFontFamilyString(ClosedCaptionStyle closed_caption_style,
       *css_font_variant = "small-caps";
       break;
     case ClosedCaptionStyle_Default:
-      *css_font_family = std::string();
+      *css_font_family = "";
       break;
   }
 }
@@ -93,7 +93,7 @@ std::string GetEdgeEffectString(ClosedCaptionEdgeEffect edge_effect) {
     case ClosedCaptionEdgeEffect_DropShadow:
       return "3px 3px 3px 2px black";
     case ClosedCaptionEdgeEffect_Default:
-      return std::string();
+      return "";
   }
 }
 
@@ -110,7 +110,7 @@ std::string GetCaptionSizeString(ClosedCaptionSize caption_size) {
     case ClosedCaptionSize_TwoHundredPercent:
       return "200%";
     case ClosedCaptionSize_Default:
-      return std::string();
+      return "";
   }
 }
 
@@ -133,8 +133,9 @@ std::string GetCssColor(ClosedCaptionColor caption_color) {
     case ClosedCaptionColor_Cyan:
       return "cyan";
     case ClosedCaptionColor_White:
-    case ClosedCaptionColor_Default:
       return "white";
+    case ClosedCaptionColor_Default:
+      return "";
   }
 }