Reland "Native window occlusion tracking for windows."

This is a reland of 03b56190ff55b876dc1b4a1398930154240a3170

Original change's description:
> Native window occlusion tracking for windows.
> 
> This implements native window occlusion tracking on windows, under flag/experiment control. It uses WinEvent event hooks and EnumWindows to track and calculate occlusion.
> 
> It is not yet hooked up to the code that treats occluded windows differently.
> 
> Bug: 813093
> Change-Id: Ic9b473b8cad38e7b1653566f28917164bc92e21c
> Reviewed-on: https://chromium-review.googlesource.com/c/1140752
> Commit-Queue: David Bienvenu <[email protected]>
> Reviewed-by: Scott Violet <[email protected]>
> Reviewed-by: François Doray <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#603619}

Bug: 813093
Change-Id: I78fa7b04a7efc0c392100b852e1a1a5f04421132
Reviewed-on: https://chromium-review.googlesource.com/c/1308264
Commit-Queue: David Bienvenu <[email protected]>
Reviewed-by: Scott Violet <[email protected]>
Cr-Commit-Position: refs/heads/master@{#604618}
diff --git a/chrome/browser/flag_descriptions.cc b/chrome/browser/flag_descriptions.cc
index e909842f..de6fb7b 100644
--- a/chrome/browser/flag_descriptions.cc
+++ b/chrome/browser/flag_descriptions.cc
@@ -2859,6 +2859,12 @@
 
 #if defined(OS_WIN)
 
+const char kCalculateNativeWinOcclusionName[] =
+    "Calculate window occlusion on Windows";
+const char kCalculateNativeWinOcclusionDescription[] =
+    "Calculate window occlusion on Windows will be used in the future "
+    "to throttle and potentially unload foreground tabs in occluded windows";
+
 const char kCloudPrintXpsName[] = "XPS in Google Cloud Print";
 const char kCloudPrintXpsDescription[] =
     "XPS enables advanced options for classic printers connected to the Cloud "