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

ash/ash_switches.cc

Index: ash/ash_switches.cc
diff --git a/ash/ash_switches.cc b/ash/ash_switches.cc
index 20fe1a8026c7cc03916bc8bb8d5cdd0f0d6a7ba9..a7075b079ce7da0df7ad07e81cabc14d6911aedd 100644
--- a/ash/ash_switches.cc
+++ b/ash/ash_switches.cc
@@ -126,6 +126,9 @@ const char kAshEnableNewAudioHandler[] = "ash-enable-new-audio-handler";
 // Enables the Oak tree viewer.
 const char kAshEnableOak[] = "ash-enable-oak";
 
+// Enables overview mode for window switching.
+const char kAshEnableOverviewMode[] = "ash-enable-overview-mode";
+
 // Enables "sticky" edges instead of "snap-to-edge"
 const char kAshEnableStickyEdges[] = "ash-enable-sticky-edges";