Add 2.0 DSF for external 4k display only if the display zoom is disabled

Display zoom is responsible for scaling when the flag is enabled so we
dont need to manualy set the device scale factor for external 4k
displays.

Bug: 851701
Change-Id: I7b2789fa233dda89d3f2dd5031b5507c4c64b1ed
Component: Display Change Observer, 4k, Scaling
Reviewed-on: https://chromium-review.googlesource.com/1096343Reviewed-by: kylechar <kylechar@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Malay Keshav <malaykeshav@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566541}

services/ui/display/screen_manager_ozone_internal_unittests.cc

@@ -254,10 +254,8 @@ TEST_F(ScreenManagerOzoneInternalTest, AddDisplay4k) {
                  .SetType(DISPLAY_CONNECTION_TYPE_DVI)
                  .Build());
 
-  // Check that display 2 has a device scale factor of 2 since it's a 4k
-  // display.
   EXPECT_EQ("Added(2)", delegate()->changes());
-  EXPECT_THAT(delegate()->added()[0], DisplayBoundsIs("1024,0 2048x1080"));
+  EXPECT_THAT(delegate()->added()[0], DisplayBoundsIs("1024,0 4096x2160"));
   EXPECT_THAT(delegate()->added()[0], DisplayPixelSizeIs("4096x2160"));
 }
 

ui/display/manager/display_change_observer.cc

@@ -293,7 +293,8 @@ ManagedDisplayInfo DisplayChangeObserver::CreateManagedDisplayInfo(
       gfx::Vector2d size_in_vec(snapshot->physical_size().width(),
                                 snapshot->physical_size().height());
       if (size_in_vec.LengthSquared() > k2xThreshouldSizeSquaredFor4KInMm &&
-          mode_info->size().width() >= kMinimumWidthFor4K) {
+          mode_info->size().width() >= kMinimumWidthFor4K &&
+          !features::IsDisplayZoomSettingEnabled()) {
         // Make sure that additional device scale factors table has 2x.
         DCHECK_EQ(2.0f, kAdditionalDeviceScaleFactorsFor4k[1]);
         device_scale_factor = 2.0f;