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 373bd1b9eb1bc3aa9353f4de5cb00d5ea3071c1b..23ed14bf396682bbbf096ba011e004f05ff396ee 100644
--- a/ash/ash_switches.cc
+++ b/ash/ash_switches.cc
@@ -114,6 +114,9 @@ const char kAshEnableMemoryMonitor[] = "ash-enable-memory-monitor";
 // 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";