Linux Aura: get tooltip background color from theme

ui/native_theme/fallback_theme.cc

Index: ui/native_theme/fallback_theme.cc
diff --git a/ui/native_theme/fallback_theme.cc b/ui/native_theme/fallback_theme.cc
index ffb15fd6626fc3bde40384e07b53e22efb19ba7d..53cffb667acb225f7fe58efe67982f1821ca771a 100644
--- a/ui/native_theme/fallback_theme.cc
+++ b/ui/native_theme/fallback_theme.cc
@@ -64,6 +64,8 @@ SkColor FallbackTheme::GetSystemColor(ColorId color_id) const {
   static const SkColor kTextfieldSelectionColor =
       color_utils::AlphaBlend(SK_ColorBLACK,
           kTextfieldSelectionBackgroundFocused, 0xdd);
+  // Tooltip
+  static const SkColor kTooltipBackground = 0xFFFFFFCC;
   // Tree
   static const SkColor kTreeBackground = SK_ColorWHITE;
   static const SkColor kTreeTextColor = SK_ColorBLACK;
@@ -180,6 +182,10 @@ SkColor FallbackTheme::GetSystemColor(ColorId color_id) const {
     case kColorId_TextfieldSelectionBackgroundFocused:
       return kTextfieldSelectionBackgroundFocused;
 
+    // Tooltip
+    case kColorId_TooltipBackground:
+      return kTooltipBackground;
+
     // Tree
     case kColorId_TreeBackground:
       return kTreeBackground;