Add "panel_fitting" GPU feature type and use it for mirror mode.

ash/ash_switches.cc

Index: ash/ash_switches.cc
diff --git a/ash/ash_switches.cc b/ash/ash_switches.cc
index 8d2dca7031e0fe19c771dd3cdf65bb4ecec84e72..7cd3691414f2af40db059af5c42b4939e8ca211a 100644
--- a/ash/ash_switches.cc
+++ b/ash/ash_switches.cc
@@ -16,6 +16,9 @@ const char kAshDebugShortcuts[] = "ash-debug-shortcuts";
 // Disables Workspace2.
 const char kAshDisableWorkspace2[] = "ash-disable-workspace2";
 
+// Disables panel fitting (used for mirror mode).
+const char kAshDisablePanelFitting[] = "ash-disable-panel-fitting";
+
 // Enable advanced gestures (e.g. for window management).
 const char kAshEnableAdvancedGestures[] = "ash-enable-advanced-gestures";