Add simplistic HLG->SDR conversion.

ui/gfx/color_transform.cc

Index: ui/gfx/color_transform.cc
diff --git a/ui/gfx/color_transform.cc b/ui/gfx/color_transform.cc
index 6db577ab5c64da8deb16771f32977b3b76aea95a..37818466d702e0214dbaf47e850f229e5d7d760f 100644
--- a/ui/gfx/color_transform.cc
+++ b/ui/gfx/color_transform.cc
@@ -574,6 +574,13 @@ class ColorSpaceToColorSpaceTransform : public ColorTransform {
           from_.transfer_ = ColorSpace::TransferID::SMPTEST2084_NON_HDR;
           break;
 
+        case ColorSpace::TransferID::ARIB_STD_B67:
+          // Interpreting HLG using a gamma 2.4 works reasonably well for SDR
+          // displays. Once we have HDR output capabilies, we'll need to
+          // change this.
+          from_.transfer_ = ColorSpace::TransferID::GAMMA24;
+          break;
+
         default:  // Do nothing
           break;
       }