Fix log spam on 10.6 due to NSWindowFullScreenButton.

NSWindowFullScreenButton is a 10.7 addition and calling
-standardWindowButton: with it on 10.6 results in log
spam of the form:

"unrecognized button type 7"

BUG=none

Review URL: https://codereview.chromium.org/905803002

Cr-Commit-Position: refs/heads/master@{#315089}

chrome/browser/ui/cocoa/browser_window_controller_private.mm

@@ -845,8 +845,9 @@ willPositionSheet:(NSWindow*)sheet
       [presentationModeController_ toolbarFraction]];
 
   [layout setHasTabStrip:[self hasTabStrip]];
-  NSButton* fullScreenButton =
-      [[self window] standardWindowButton:NSWindowFullScreenButton];
+  // NSWindowFullScreenButton is 10.7+ and results in log spam on 10.6 if used.
+  NSButton* fullScreenButton = base::mac::IsOSSnowLeopard() ?
+      nil : [[self window] standardWindowButton:NSWindowFullScreenButton];
   [layout setFullscreenButtonFrame:fullScreenButton ? [fullScreenButton frame]
                                                     : NSZeroRect];
   if ([self shouldShowAvatar]) {