Force constrained windows from nibs to be borderless

chrome/browser/ui/cocoa/constrained_window/constrained_window_custom_window.mm

Index: chrome/browser/ui/cocoa/constrained_window/constrained_window_custom_window.mm
diff --git a/chrome/browser/ui/cocoa/constrained_window/constrained_window_custom_window.mm b/chrome/browser/ui/cocoa/constrained_window/constrained_window_custom_window.mm
index 6c66203f6d944d8c4c3cbd12b458c4d92da0ceda..be09b79bb4c3672e5dc6abe8942331cdb29e48da 100644
--- a/chrome/browser/ui/cocoa/constrained_window/constrained_window_custom_window.mm
+++ b/chrome/browser/ui/cocoa/constrained_window/constrained_window_custom_window.mm
@@ -30,7 +30,7 @@
                   backing:(NSBackingStoreType)bufferingType
                     defer:(BOOL)deferCreation {
   if ((self = [super initWithContentRect:contentRect
-                               styleMask:windowStyle
+                               styleMask:NSBorderlessWindowMask
                                  backing:bufferingType
                                    defer:NO])) {
     [self setHasShadow:YES];