Add window overview mode behind --ash-enable-overview-mode flag to F5 key.

chrome/app/generated_resources.grd

Index: chrome/app/generated_resources.grd
diff --git a/chrome/app/generated_resources.grd b/chrome/app/generated_resources.grd
index 6378b4a6377d301622b2241856eb2b032602e71c..b36e6c855fa998d7e0d13cd173bb81048e234062 100644
--- a/chrome/app/generated_resources.grd
+++ b/chrome/app/generated_resources.grd
@@ -6677,6 +6677,12 @@ Keep your key file in a safe place. You will need it to create new versions of y
       <message name="IDS_FLAGS_OVERSCROLL_HISTORY_NAVIGATION_DESCRIPTION" desc="Description for the flag to disable history navigation from horizontal overscroll.">
         Experimental history navigation in response to horizontal overscroll.
       </message>
+      <message name="IDS_FLAGS_OVERVIEW_MODE_NAME" desc="Title for the flag to enable window overview mode.">
+        Enable overview mode.
+      </message>
+      <message name="IDS_FLAGS_OVERVIEW_MODE_DESCRIPTION" desc="Description for the flag to enable window overview mode.">
+        Enable overview mode, activated by pushing the switch window button.
+      </message>
       <message name="IDS_FLAGS_ENABLE_SYNC_FAVICONS_NAME" desc="Title for the flag to enable the favicon sync datatype">
         Enable favicon sync.
       </message>