color: Clarify default behaviors

ui/gfx/color_transform_unittest.cc

Index: ui/gfx/color_transform_unittest.cc
diff --git a/ui/gfx/color_transform_unittest.cc b/ui/gfx/color_transform_unittest.cc
index 5a8882672f7ffb9933581876b9e30bc86ba7d9ae..5d4de4937c05b8f68152af9b6cae9724c8f52eb0 100644
--- a/ui/gfx/color_transform_unittest.cc
+++ b/ui/gfx/color_transform_unittest.cc
@@ -194,8 +194,10 @@ TEST(SimpleColorSpace, GetColorSpace) {
   EXPECT_NEAR(tmp.z(), 1.0f, kEpsilon);
 }
 
-TEST(SimpleColorSpace, UnknownToSRGB) {
-  ColorSpace unknown;
+TEST(SimpleColorSpace, UnknownVideoToSRGB) {
+  // Invalid video spaces should be BT709.
+  ColorSpace unknown = gfx::ColorSpace::CreateVideo(
+      -1, -1, -1, gfx::ColorSpace::RangeID::LIMITED);
   ColorSpace sRGB = ColorSpace::CreateSRGB();
   std::unique_ptr<ColorTransform> t(ColorTransform::NewColorTransform(
       unknown, sRGB, ColorTransform::Intent::INTENT_PERCEPTUAL));
@@ -219,6 +221,19 @@ TEST(SimpleColorSpace, UnknownToSRGB) {
   EXPECT_GT(tmp.z(), tmp.y());
 }
 
+TEST(SimpleColorSpace, DefaultToSRGB) {
+  // The default value should do no transformation, regardless of destination.
+  ColorSpace unknown;
+  std::unique_ptr<ColorTransform> t1(ColorTransform::NewColorTransform(
+      unknown, ColorSpace::CreateSRGB(),
+      ColorTransform::Intent::INTENT_PERCEPTUAL));
+  EXPECT_EQ(t1->NumberOfStepsForTesting(), 0u);
+  std::unique_ptr<ColorTransform> t2(ColorTransform::NewColorTransform(
+      unknown, ColorSpace::CreateXYZD50(),
+      ColorTransform::Intent::INTENT_PERCEPTUAL));
+  EXPECT_EQ(t2->NumberOfStepsForTesting(), 0u);
+}
+
 class TransferTest : public testing::TestWithParam<ColorSpace::TransferID> {};
 
 TEST_P(TransferTest, basicTest) {