Mac: Don't update prefs file unnecessarily when activating windows.

When activating Chrome app from another app, often times we are reactivating
the same browser window. In this case, we don't need to update prefs with
a new profile and window position. Avoiding prefs update is good because it
triggers JSON file writing to disk (after a 10 seconds delay).

BUG=482027

Review URL: https://codereview.chromium.org/1105423003

Cr-Commit-Position: refs/heads/master@{#327418}

chrome/browser/ui/cocoa/browser_window_controller.mm

@@ -55,6 +55,7 @@
 #import "chrome/browser/ui/cocoa/framed_browser_window.h"
 #import "chrome/browser/ui/cocoa/fullscreen_window.h"
 #import "chrome/browser/ui/cocoa/infobars/infobar_container_controller.h"
+#include "chrome/browser/ui/cocoa/last_active_browser_cocoa.h"
 #import "chrome/browser/ui/cocoa/location_bar/autocomplete_text_field_editor.h"
 #import "chrome/browser/ui/cocoa/presentation_mode_controller.h"
 #import "chrome/browser/ui/cocoa/profiles/avatar_base_controller.h"
@@ -590,8 +591,10 @@ using content::WebContents;
 
 // Called right after our window became the main window.
 - (void)windowDidBecomeMain:(NSNotification*)notification {
-  BrowserList::SetLastActive(browser_.get());
-  [self saveWindowPositionIfNeeded];
+  if (chrome::GetLastActiveBrowser() != browser_) {
+    BrowserList::SetLastActive(browser_.get());
+    [self saveWindowPositionIfNeeded];
+  }
 
   [[[self window] contentView] cr_recursivelyInvokeBlock:^(id view) {
       if ([view conformsToProtocol:@protocol(ThemedWindowDrawing)])