ui/display: pump up HDR level for ChromeOS

This CL cranks up the HDR level multiplier from 3x to 4x so that the
bright areas of the videos are not overexposed, and maintain local
contrast. Tested on kohaku and my LG 4K HDR monitor.

Bug: 958166
Change-Id: I88d7661340c43c98f5bad43bce30e9f58142cd92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2324949
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Auto-Submit: Miguel Casas <mcasas@chromium.org>
Reviewed-by: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792448}

ui/display/manager/display_change_observer.cc

@@ -98,7 +98,7 @@ gfx::DisplayColorSpaces FillDisplayColorSpaces(
 
   if (allow_high_bit_depth && snapshot_color_space.IsHDR()) {
     constexpr float kSDRJoint = 0.75;
-    constexpr float kHDRLevel = 3.0;
+    constexpr float kHDRLevel = 4.0;
     const auto primary_id = snapshot_color_space.GetPrimaryID();
     gfx::ColorSpace hdr_color_space;
     if (primary_id == gfx::ColorSpace::PrimaryID::CUSTOM) {