Add an experimental keyboard lock, where Esc must be held to exit.

chrome/browser/about_flags.cc

Index: chrome/browser/about_flags.cc
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 721e6657ab00b330e97d064fbe6c146d10c4edbf..89ceb0730336141a109556dcfdfb543d579ac7c8 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -1595,6 +1595,9 @@ const FeatureEntry kFeatureEntries[] = {
     {"simplified-fullscreen-ui", IDS_FLAGS_SIMPLIFIED_FULLSCREEN_UI_NAME,
      IDS_FLAGS_SIMPLIFIED_FULLSCREEN_UI_DESCRIPTION, kOsAll,
      FEATURE_VALUE_TYPE(features::kSimplifiedFullscreenUI)},
+    {"experimental-fullscreen-ui", IDS_FLAGS_EXPERIMENTAL_FULLSCREEN_UI_NAME,
+     IDS_FLAGS_EXPERIMENTAL_FULLSCREEN_UI_DESCRIPTION, kOsDesktop,
+     FEATURE_VALUE_TYPE(features::kExperimentalFullscreenUI)},
 #if defined(OS_ANDROID)
     {"progress-bar-animation", IDS_FLAGS_PROGRESS_BAR_ANIMATION_NAME,
      IDS_FLAGS_PROGRESS_BAR_ANIMATION_DESCRIPTION, kOsAndroid,