Don't put a text shadow on blue buttons in gtk theme mode.

ui/native_theme/common_theme.cc

Index: ui/native_theme/common_theme.cc
diff --git a/ui/native_theme/common_theme.cc b/ui/native_theme/common_theme.cc
index d1797f9eb2340332ebff81ce3ae3271d3f18cd26..0864f499b76b0742cae488b97d3c3c2d2b0b0771 100644
--- a/ui/native_theme/common_theme.cc
+++ b/ui/native_theme/common_theme.cc
@@ -39,6 +39,7 @@ const SkColor kBlueButtonEnabledColor = SK_ColorWHITE;
 const SkColor kBlueButtonDisabledColor = SK_ColorWHITE;
 const SkColor kBlueButtonPressedColor = SK_ColorWHITE;
 const SkColor kBlueButtonHoverColor = SK_ColorWHITE;
+const SkColor kBlueButtonShadowColor = SkColorSetRGB(0x53, 0x8C, 0xEA);
 
 }  // namespace
 
@@ -101,6 +102,8 @@ bool CommonThemeGetSystemColor(NativeTheme::ColorId color_id, SkColor* color) {
       break;
     case NativeTheme::kColorId_BlueButtonHoverColor:
       *color = kBlueButtonHoverColor;
+    case NativeTheme::kColorId_BlueButtonShadowColor:
+      *color = kBlueButtonShadowColor;
       break;
     default:
       return false;