helium/ui: add MRU tab cycling popup behind experimental flag

i18n/source.gen.json

@@ -575,6 +575,24 @@
     "context": "Description for the toggle to pin the top bar when in Zen mode.",
     "message": "Always show the top bar"
   },
+  {
+    "name": "IDS_TAB_CYCLING_POPUP_ACCESSIBLE_NAME",
+    "source": "chrome/app/settings_strings.grdp",
+    "context": "Screen reader label for the MRU tab cycling popup.",
+    "message": "Recently used tabs"
+  },
+  {
+    "name": "IDS_TAB_CYCLING_AND_MORE_TABS",
+    "source": "chrome/app/settings_strings.grdp",
+    "context": "Footer shown at the bottom of the MRU tab cycling popup when additional tabs are not visible.",
+    "message": "{COUNT, plural,=1 {and 1 more...}other {and # more...}}"
+  },
+  {
+    "name": "IDS_TAB_CYCLING_UNTITLED_TAB",
+    "source": "chrome/app/settings_strings.grdp",
+    "context": "Fallback label in the MRU tab cycling popup when a tab has no title.",
+    "message": "Untitled"
+  },
   {
     "name": "IDS_SETTINGS_DEFAULT_BROWSER_SECONDARY",
     "source": "chrome/app/settings_chromium_strings.grdp",

patches/helium/ui/experiments/tab-cycling-popup-wiring.patch

@@ -0,0 +1,50 @@
+--- a/chrome/browser/ui/ui_features.cc
++++ b/chrome/browser/ui/ui_features.cc
+@@ -32,6 +32,12 @@ bool IsHeliumZenModeFeatureEnabled() {
+   return base::FeatureList::IsEnabled(kHeliumZenMode);
+ }
+
++BASE_FEATURE(kHeliumTabCyclingPopup, base::FEATURE_DISABLED_BY_DEFAULT);
++
++bool IsHeliumTabCyclingPopupFeatureEnabled() {
++  return base::FeatureList::IsEnabled(kHeliumTabCyclingPopup);
++}
++
+ // Enables the use of WGC for the Eye Dropper screen capture.
+ BASE_FEATURE(kAllowEyeDropperWGCScreenCapture,
+ #if BUILDFLAG(IS_WIN)
+--- a/chrome/browser/ui/ui_features.h
++++ b/chrome/browser/ui/ui_features.h
+@@ -25,6 +25,9 @@ bool HeliumUseCompactLocationWidth();
+ BASE_DECLARE_FEATURE(kHeliumZenMode);
+ bool IsHeliumZenModeFeatureEnabled();
+
++BASE_DECLARE_FEATURE(kHeliumTabCyclingPopup);
++bool IsHeliumTabCyclingPopupFeatureEnabled();
++
+ BASE_DECLARE_FEATURE(kAllowEyeDropperWGCScreenCapture);
+
+ BASE_DECLARE_FEATURE(kBrowserWidgetCacheThemeService);
+--- a/chrome/browser/helium_flag_choices.h
++++ b/chrome/browser/helium_flag_choices.h
+@@ -38,6 +38,8 @@ namespace helium {
+   constexpr const char kHeliumCompactLocationWidthCommandLine[] =
+       "helium-compact-location-width";
+   constexpr const char kHeliumZenModeCommandLine[] = "helium-zen-mode";
++  constexpr const char kHeliumTabCyclingPopupCommandLine[] =
++      "helium-tab-cycling-popup";
+ }  // namespace helium
+
+ #endif  /* CHROME_BROWSER_HELIUM_FLAG_CHOICES_H_ */
+--- a/chrome/browser/helium_flag_entries.h
++++ b/chrome/browser/helium_flag_entries.h
+@@ -46,4 +46,9 @@
+      "Hides browser chrome (toolbar, tab strip, bookmarks bar) and reveals it "
+      "on hover near the window edges. Helium flag.",
+      kOsDesktop, FEATURE_VALUE_TYPE(features::kHeliumZenMode)},
++    {helium::kHeliumTabCyclingPopupCommandLine,
++     "Tab Cycling Popup",
++     "Shows a popup to select which tab to switch to when cycling tabs "
++     "in most recently used order. Helium flag.",
++     kOsDesktop, FEATURE_VALUE_TYPE(features::kHeliumTabCyclingPopup)},
+ #endif  /* CHROME_BROWSER_HELIUM_FLAG_ENTRIES_H_ */