Make tooltips toplevel windows on Linux

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 3e7b350346fe1a496abd82318751d56b45b73270..d995d71c69af5a1946c54ff702bb6b946ed595da 100644
--- a/ui/native_theme/fallback_theme.cc
+++ b/ui/native_theme/fallback_theme.cc
@@ -68,6 +68,7 @@ SkColor FallbackTheme::GetSystemColor(ColorId color_id) const {
           kTextfieldSelectionBackgroundFocused, 0xdd);
   // Tooltip
   static const SkColor kTooltipBackground = 0xFFFFFFCC;
+  static const SkColor kTooltipTextColor = kLabelEnabledColor;
   // Tree
   static const SkColor kTreeBackground = SK_ColorWHITE;
   static const SkColor kTreeTextColor = SK_ColorBLACK;
@@ -189,6 +190,8 @@ SkColor FallbackTheme::GetSystemColor(ColorId color_id) const {
     // Tooltip
     case kColorId_TooltipBackground:
       return kTooltipBackground;
+    case kColorId_TooltipText:
+      return kTooltipTextColor;
 
     // Tree
     case kColorId_TreeBackground: