views: refactor away PlatformStyle::BackgroundColorForMdButton

This change:
1) Introduces color_utils::ImplicitAlphaBlend, which does an alpha
   blend using the alpha of the foreground color;
2) Introduces a NativeTheme color ID for the shading of a pressed
   button;
3) Uses ImplicitAlphaBlend to blend that shade, instead of calling
   into PlatformStyle;
4) Deletes PlatformStyle::BackgroundColorForMdButton

BUG=644543

Review-Url: https://codereview.chromium.org/2319313003
Cr-Commit-Position: refs/heads/master@{#417366}

chrome/browser/ui/libgtk2ui/native_theme_gtk2.cc

@@ -329,6 +329,8 @@ SkColor NativeThemeGtk2::GetSystemColor(ColorId color_id) const {
       return GetSystemColor(kColorId_LinkEnabled);
     case kColorId_TextOnCallToActionColor:
       return GetTextColor(GetLabel(), SELECTED);
+    case kColorId_ButtonPressedShade:
+      return SK_ColorTRANSPARENT;
 
     // Textfield
     case kColorId_TextfieldDefaultColor:

ui/gfx/color_utils.cc

@@ -284,6 +284,11 @@ SkColor AlphaBlend(SkColor foreground, SkColor background, SkAlpha alpha) {
                         static_cast<int>(std::round(b)));
 }
 
+SkColor GetResultingPaintColor(SkColor foreground, SkColor background) {
+  return AlphaBlend(SkColorSetA(foreground, SK_AlphaOPAQUE), background,
+                    SkColorGetA(foreground));
+}
+
 bool IsDark(SkColor color) {
   return GetLuma(color) < 128;
 }

ui/gfx/color_utils.h

@@ -94,6 +94,11 @@ GFX_EXPORT double CalculateBoringScore(const SkBitmap& bitmap);
 GFX_EXPORT SkColor AlphaBlend(SkColor foreground, SkColor background,
                               SkAlpha alpha);
 
+// Returns the color that results from painting |foreground| on top of
+// |background|.
+GFX_EXPORT SkColor GetResultingPaintColor(SkColor foreground,
+                                          SkColor background);
+
 // Returns true if the luma of |color| is closer to black than white.
 GFX_EXPORT bool IsDark(SkColor color);
 

ui/native_theme/common_theme.cc

@@ -155,6 +155,7 @@ SkColor GetAuraColor(NativeTheme::ColorId color_id,
   static const SkColor kBlueButtonShadowColor = SkColorSetRGB(0x53, 0x8C, 0xEA);
   static const SkColor kCallToActionColor = gfx::kGoogleBlue500;
   static const SkColor kTextOnCallToActionColor = SK_ColorWHITE;
+  static const SkColor kButtonPressedShade = SK_ColorTRANSPARENT;
   // MenuItem:
   static const SkColor kMenuBackgroundColor = SK_ColorWHITE;
   static const SkColor kMenuHighlightBackgroundColor =
@@ -291,6 +292,8 @@ SkColor GetAuraColor(NativeTheme::ColorId color_id,
       return kCallToActionColor;
     case NativeTheme::kColorId_TextOnCallToActionColor:
       return kTextOnCallToActionColor;
+    case NativeTheme::kColorId_ButtonPressedShade:
+      return kButtonPressedShade;
 
     // MenuItem
     case NativeTheme::kColorId_MenuBorderColor:

ui/native_theme/native_theme.h

@@ -259,6 +259,7 @@ class NATIVE_THEME_EXPORT NativeTheme {
     kColorId_ButtonHighlightColor,
     kColorId_ButtonHoverColor,
     kColorId_ButtonHoverBackgroundColor,
+    kColorId_ButtonPressedShade,
     kColorId_BlueButtonEnabledColor,
     kColorId_BlueButtonDisabledColor,
     kColorId_BlueButtonPressedColor,

ui/native_theme/native_theme_dark_aura.cc

@@ -91,6 +91,7 @@ SkColor NativeThemeDarkAura::GetSystemColor(ColorId color_id) const {
 
     // Intentional pass-throughs to NativeThemeAura.
     case kColorId_TextOnCallToActionColor:
+    case kColorId_ButtonPressedShade:
     case kColorId_ResultsTableHoveredBackground:
     case kColorId_ResultsTableSelectedBackground:
     case kColorId_ResultsTableNormalUrl:

ui/native_theme/native_theme_mac.mm

@@ -126,7 +126,8 @@ SkColor NativeThemeMac::ApplySystemControlTint(SkColor color) {
 }
 
 SkColor NativeThemeMac::GetSystemColor(ColorId color_id) const {
-  // Even with --secondary-ui-md, menus use the platform colors and styling.
+  // Even with --secondary-ui-md, menus use the platform colors and styling, and
+  // Mac has a couple of specific color overrides.
   switch (color_id) {
     case kColorId_EnabledMenuItemForegroundColor:
       return NSSystemColorToSkColor([NSColor controlTextColor]);
@@ -145,6 +146,9 @@ SkColor NativeThemeMac::GetSystemColor(ColorId color_id) const {
                                    : kMenuSeparatorColor;
     case kColorId_MenuBorderColor:
       return kMenuBorderColor;
+
+    case kColorId_ButtonPressedShade:
+      return SkColorSetA(SK_ColorBLACK, 0x08);
     default:
       break;
   }

ui/views/controls/button/md_text_button.cc

@@ -312,9 +312,11 @@ void MdTextButton::UpdateColors() {
     bg_color = color_utils::BlendTowardOppositeLuma(text_color, 0xD8);
   }
 
-  // TODO(ellyjones): Excise this, in favor of a helper function wrapping
-  // |color_utils::AlphaBlend| and a new NativeTheme color constant.
-  bg_color = PlatformStyle::BackgroundColorForMdButton(bg_color, state());
+  if (state() == STATE_PRESSED) {
+    SkColor shade =
+        theme->GetSystemColor(ui::NativeTheme::kColorId_ButtonPressedShade);
+    bg_color = color_utils::GetResultingPaintColor(shade, bg_color);
+  }
 
   const SkAlpha kStrokeOpacity = 0x1A;
   SkColor stroke_color = (is_cta_ || color_utils::IsDark(text_color))

ui/views/style/platform_style.cc

@@ -89,12 +89,6 @@ SkColor PlatformStyle::TextColorForButton(
   return color_by_state[button.state()];
 }
 
-SkColor PlatformStyle::BackgroundColorForMdButton(
-    SkColor color,
-    Button::ButtonState state) {
-  return color;
-}
-
 #endif  // OS_MACOSX
 
 #if !defined(DESKTOP_LINUX) && !defined(OS_MACOSX)

ui/views/style/platform_style.h

@@ -88,11 +88,6 @@ class VIEWS_EXPORT PlatformStyle {
   static void ApplyLabelButtonTextStyle(Label* label,
                                         ButtonColorByState* color_by_state);
 
-  // Returns the background color that should be used for an MdTextButton or
-  // other MD controls when in the given state.
-  static SkColor BackgroundColorForMdButton(SkColor color,
-                                            Button::ButtonState state);
-
   // Applies the current system theme to the default border created by |button|.
   static std::unique_ptr<Border> CreateThemedLabelButtonBorder(
       LabelButton* button);

ui/views/style/platform_style_mac.mm

@@ -96,14 +96,4 @@ void PlatformStyle::ApplyLabelButtonTextStyle(
       theme->GetSystemColor(ui::NativeTheme::kColorId_ButtonHighlightColor);
 }
 
-// static
-SkColor PlatformStyle::BackgroundColorForMdButton(
-    SkColor color,
-    Button::ButtonState state) {
-  // Per Harmony specs: Pressed state on Mac is + #000 at 0.05 alpha.
-  if (state == Button::STATE_PRESSED)
-    return color_utils::AlphaBlend(SK_ColorBLACK, color, 0x08);
-  return color;
-}
-
 }  // namespace views