dpwas: Add chrome://flags entry for Window Controls Overlay

Explainer: https://github.com/WICG/window-controls-overlay/blob/main/explainer.md
Design Doc: https://docs.google.com/document/d/1k0YL_-VMLIfjYCgJ2v6cMvuUv2qMKg4BgLI2tJ4qtyo/edit?usp=sharing
I2P: https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/cper6nNLFRQ/hU91kfCWBQAJ

Bug: 937121
Change-Id: I7772b80d3c415b03c67b7db9379ebe59a0da092a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2780941
Reviewed-by: Chase Phillips <[email protected]>
Commit-Queue: Amanda Baker <[email protected]>
Cr-Commit-Position: refs/heads/master@{#866200}
diff --git a/chrome/browser/flag_descriptions.cc b/chrome/browser/flag_descriptions.cc
index 38bd9a0..1c22ff2 100644
--- a/chrome/browser/flag_descriptions.cc
+++ b/chrome/browser/flag_descriptions.cc
@@ -741,6 +741,13 @@
     "implementation of: "
     "https://github.com/WICG/pwa-url-handler/blob/master/explainer.md";
 
+const char kDesktopPWAsWindowControlsOverlayName[] =
+    "Desktop PWA Window Controls Overlay";
+const char kDesktopPWAsWindowControlsOverlayDescription[] =
+    "Enable web app manifests to declare Window Controls Overlay as a display "
+    "override. Prototype implementation of: "
+    "https://github.com/WICG/window-controls-overlay/blob/main/explainer.md";
+
 const char kEnableMigrateDefaultChromeAppToWebAppsGSuiteName[] =
     "Migrate default G Suite Chrome apps to web apps";
 const char kEnableMigrateDefaultChromeAppToWebAppsGSuiteDescription[] =