[md] Fixes tooltips on Linux that should not have changed

ui/views/corewm/tooltip_aura.cc

Index: ui/views/corewm/tooltip_aura.cc
diff --git a/ui/views/corewm/tooltip_aura.cc b/ui/views/corewm/tooltip_aura.cc
index f33eaca7d4084325afd9d1a70ebb42f21d475687..a0d25bfc84df1e8173d4093bb43118294f946347 100644
--- a/ui/views/corewm/tooltip_aura.cc
+++ b/ui/views/corewm/tooltip_aura.cc
@@ -29,9 +29,6 @@ namespace {
 // be wrapped.
 const int kTooltipMaxWidthPixels = 400;
 
-// Corner radius of tooltip background used with Material Design.
-const float kTooltipCornerRadius = 2.f;
-
 // FIXME: get cursor offset from actual cursor size.
 const int kCursorOffsetX = 10;
 const int kCursorOffsetY = 15;
@@ -119,12 +116,21 @@ class TooltipAura::TooltipView : public views::View {
   }
 
   void SetBackgroundColor(SkColor background_color) {
+    // TODO(varkha): Update if native widget can be transparent on Linux.
+#if defined(OS_LINUX) && !defined(OS_CHROMEOS)
+    views::Background* background =
+        views::Background::CreateSolidBackground(background_color);
+#else
+    // Corner radius of tooltip background used with Material Design.
+    const float kTooltipCornerRadius = 2.f;
+    const bool material = ui::MaterialDesignController::IsModeMaterial();
     views::Background* background =
-        ui::MaterialDesignController::IsModeMaterial()
+        material
             ? views::Background::CreateBackgroundPainter(
                   true, views::Painter::CreateSolidRoundRectPainter(
                             background_color, kTooltipCornerRadius))
             : views::Background::CreateSolidBackground(background_color);
+#endif
     set_background(background);
     // Force the text color to be readable when |background_color| is not
     // opaque.