Remove MutableStyle from dark-mode unittests.

I don't believe(?) there is actually a way to set the opacity on a
viewport. (Either with the @viewport rule, or the style fixup[1]).

This removes one of the unittests, and removes the logic which
was reading the opacity from the viewport.

[1] https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/dom/document.cc?q=viewport_style&sq=package:chromium&dr=C&l=2122

Bug: 813068
Change-Id: I29d1b45c848c3ddca48a04523ec358094e97755b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1637044Reviewed-by: Aran Gilman <gilmanmh@google.com>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665148}

third_party/blink/renderer/core/accessibility/apply_dark_mode.cc

@@ -21,7 +21,7 @@ bool HasLightBackground(const LayoutView& root) {
   const ComputedStyle& style = root.StyleRef();
   if (style.HasBackground()) {
     Color color = style.VisitedDependentColor(GetCSSPropertyBackgroundColor());
-    return IsLight(color, style.Opacity());
+    return IsLight(color);
   }
 
   // If we can't easily determine the background color, default to inverting the

third_party/blink/renderer/core/accessibility/apply_dark_mode_test.cc

@@ -29,9 +29,6 @@ TEST_F(ApplyDarkModeCheckTest, LightSolidBackgroundAlwaysFiltered) {
 TEST_F(ApplyDarkModeCheckTest, DarkSolidBackgroundFilteredIfPolicyIsFilterAll) {
   GetDocument().body()->SetInlineStyleProperty(CSSPropertyID::kBackgroundColor,
                                                CSSValueID::kBlack);
-  // TODO(https://crbug.com/925949): Set opacity the same way as the other CSS
-  // properties.
-  GetLayoutView().MutableStyle()->SetOpacity(0.9);
   UpdateAllLifecyclePhasesForTest();
 
   EXPECT_FALSE(ShouldApplyDarkModeFilterToPage(
@@ -40,20 +37,6 @@ TEST_F(ApplyDarkModeCheckTest, DarkSolidBackgroundFilteredIfPolicyIsFilterAll) {
                                               GetLayoutView()));
 }
 
-TEST_F(ApplyDarkModeCheckTest, DarkLowOpacityBackgroundAlwaysFiltered) {
-  GetDocument().body()->SetInlineStyleProperty(CSSPropertyID::kBackgroundColor,
-                                               CSSValueID::kBlack);
-  // TODO(https://crbug.com/925949): Set opacity the same way as the other CSS
-  // properties.
-  GetLayoutView().MutableStyle()->SetOpacity(0.1);
-  UpdateAllLifecyclePhasesForTest();
-
-  EXPECT_TRUE(ShouldApplyDarkModeFilterToPage(
-      DarkModePagePolicy::kFilterByBackground, GetLayoutView()));
-  EXPECT_TRUE(ShouldApplyDarkModeFilterToPage(DarkModePagePolicy::kFilterAll,
-                                              GetLayoutView()));
-}
-
 TEST_F(ApplyDarkModeCheckTest, DarkTransparentBackgroundAlwaysFiltered) {
   GetDocument().body()->SetInlineStyleProperty(CSSPropertyID::kBackgroundColor,
                                                CSSValueID::kTransparent);

third_party/blink/renderer/platform/graphics/dark_mode_color_classifier.cc

@@ -13,12 +13,11 @@ namespace blink {
 const float kOpacityThreshold = 0.4;
 
 // TODO(https://crbug.com/925949): Find a better algorithm for this.
-bool IsLight(const Color& color, float opacity) {
-  // Multiply the opacity by the alpha value to get a more accurate sense of how
-  // transparent the element is.
-  float real_opacity = opacity * (color.Alpha() / 255);
+bool IsLight(const Color& color) {
+  // Use the alpha value as the opacity.
+  float opacity = (color.Alpha() / 255);
   // Assume the color is light if the background is sufficiently transparent.
-  if (real_opacity < kOpacityThreshold) {
+  if (opacity < kOpacityThreshold) {
     return true;
   }
   double hue;

third_party/blink/renderer/platform/graphics/dark_mode_color_classifier.h

@@ -10,7 +10,7 @@
 
 namespace blink {
 
-bool PLATFORM_EXPORT IsLight(const Color& color, float opacity);
+bool PLATFORM_EXPORT IsLight(const Color& color);
 
 }  // namespace blink