Mac fullscreen low power: Add unittests

chrome/browser/ui/cocoa/browser_window_controller_private.mm

Index: chrome/browser/ui/cocoa/browser_window_controller_private.mm
diff --git a/chrome/browser/ui/cocoa/browser_window_controller_private.mm b/chrome/browser/ui/cocoa/browser_window_controller_private.mm
index b6ce3acc059d881b2597ef04cee0481b18c728ad..1c9c69e72106d753329d9b5b305f05c81ced564f 100644
--- a/chrome/browser/ui/cocoa/browser_window_controller_private.mm
+++ b/chrome/browser/ui/cocoa/browser_window_controller_private.mm
@@ -1195,14 +1195,20 @@ - (NSArray*)customWindowsToEnterFullScreenForWindow:(NSWindow*)window {
   if (![self shouldUseCustomAppKitFullscreenTransition:YES])
     return nil;
 
-  WebContents* webContents = [self webContents];
   NSWindow* lowPowerWindow = nil;
-  if (webContents) {
-    fullscreenLowPowerCoordinator_.reset(new FullscreenLowPowerCoordinatorCocoa(
-        [self window],
-        webContents->GetRenderWidgetHostView()->GetAcceleratedWidgetMac()));
-    lowPowerWindow =
-        fullscreenLowPowerCoordinator_->GetFullscreenLowPowerWindow();
+  static const bool fullscreen_low_power_enabled_at_command_line =
+      base::CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kEnableFullscreenLowPowerMode);
+  if (fullscreen_low_power_enabled_at_command_line) {
+    WebContents* webContents = [self webContents];
+    if (webContents && webContents->GetRenderWidgetHostView()) {
+      fullscreenLowPowerCoordinator_.reset(
+          new FullscreenLowPowerCoordinatorCocoa(
+              [self window], webContents->GetRenderWidgetHostView()
+                                 ->GetAcceleratedWidgetMac()));
+      lowPowerWindow =
+          fullscreenLowPowerCoordinator_->GetFullscreenLowPowerWindow();
+    }
   }
 
   fullscreenTransition_.reset(