Remove obsolete histogram Windows.ComputeNativeWindowOcclusionTime.

Fixed: 1066090
Change-Id: If351197bddae9eae2a0cef42cf01d6ad8a01412b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2517705
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: David Bienvenu <davidbienvenu@chromium.org>
Reviewed-by: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Auto-Submit: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824073}

tools/metrics/histograms/histograms_xml/windows/histograms.xml

@@ -43,6 +43,10 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="Windows.ComputeNativeWindowOcclusionTime" units="microseconds"
     expires_after="M83">
+  <obsolete>
+    Removed 11/2020. As of June 15, 2020, data indicates that computing
+    occlusion is fast enough at all percentiles.
+  </obsolete>
   <owner>fdoray@chromium.org</owner>
   <summary>
     The amount of CPU time a call to ComputeNativeWindowOcclusionStatus() takes.

ui/aura_extra/window_occlusion_impl_win.cc

@@ -4,7 +4,6 @@
 
 #include "ui/aura_extra/window_occlusion_impl_win.h"
 
-#include "base/metrics/histogram_macros.h"
 #include "base/win/scoped_gdi_object.h"
 #include "ui/aura/window_tree_host.h"
 #include "ui/gfx/geometry/rect.h"
@@ -174,7 +173,14 @@ ComputeNativeWindowOcclusionStatusImpl(
     const std::vector<aura::WindowTreeHost*>& windows,
     std::unique_ptr<NativeWindowIterator> iterator,
     std::unique_ptr<WindowBoundsDelegate> bounds_delegate) {
-  base::TimeTicks calculation_start_time = base::TimeTicks::Now();
+  // Time to execute this method, according to 28 days of Stable data
+  // ending on June 15, 2020:
+  //
+  //  50th percentile: 156 us
+  //  75th percentile: 273 us
+  //  95th percentile: 647 us
+  //  99th percentile: 1939 us
+  //  99.5th percentile: 5592 us
 
   WindowEvaluatorImpl window_evaluator(windows, std::move(bounds_delegate));
 
@@ -182,12 +188,6 @@ ComputeNativeWindowOcclusionStatusImpl(
   if (window_evaluator.HasAtLeastOneVisibleWindow())
     iterator->Iterate(&window_evaluator);
 
-  UMA_HISTOGRAM_CUSTOM_MICROSECONDS_TIMES(
-      "Windows.ComputeNativeWindowOcclusionTime",
-      base::TimeTicks::Now() - calculation_start_time,
-      base::TimeDelta::FromMicroseconds(1), base::TimeDelta::FromSeconds(10),
-      50);
-
   return window_evaluator.TakeResult();
 }