base::mac::IsOSSierra() -> base::mac::IsOS10_12(), etc.

chrome/browser/ui/cocoa/browser_window_controller.mm

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "chrome/browser/ui/cocoa/browser_window_controller.h"
 
 #include <cmath>
 #include <numeric>
 #include <utility>
 
@@ skipping 1923 matching lines @@
   return [avatarButtonController_ view];
 }
 
 - (void)enterWebContentFullscreen {
   // HTML5 Fullscreen should only use AppKit fullscreen in 10.10+.
   // However, if the user is using multiple monitors and turned off
   // "Separate Space in Each Display", use Immersive Fullscreen so
   // that the other monitors won't blank out.
   display::Screen* screen = display::Screen::GetScreen();
   BOOL hasMultipleMonitors = screen && screen->GetNumDisplays() > 1;
-  if (base::mac::IsOSYosemiteOrLater() &&
+  if (base::mac::IsAtLeastOS10_10() &&
       !(hasMultipleMonitors && ![NSScreen screensHaveSeparateSpaces])) {
     [self enterAppKitFullscreen];
   } else {
     [self enterImmersiveFullscreen];
   }
 
   if (!exclusiveAccessController_->url().is_empty())
     [self updateFullscreenExitBubble];
 }
 
@@ skipping 95 matching lines @@
 
 - (NSRect)savedRegularWindowFrame {
   return savedRegularWindowFrame_;
 }
 
 - (BOOL)isFullscreenTransitionInProgress {
   return enteringAppKitFullscreen_ || exitingAppKitFullscreen_;
 }
 
 @end  // @implementation BrowserWindowController(WindowType)