Fix user manager window's rounded corners by ensuring its content view is layer backed.

chrome/browser/ui/cocoa/profiles/user_manager_mac.mm

Index: chrome/browser/ui/cocoa/profiles/user_manager_mac.mm
diff --git a/chrome/browser/ui/cocoa/profiles/user_manager_mac.mm b/chrome/browser/ui/cocoa/profiles/user_manager_mac.mm
index 261283b4dcd82ad7822c5c49297551310e1ac207..5f850aed19f01b36cfe26374433a62b30371ca82 100644
--- a/chrome/browser/ui/cocoa/profiles/user_manager_mac.mm
+++ b/chrome/browser/ui/cocoa/profiles/user_manager_mac.mm
@@ -316,6 +316,14 @@ class UserManagerProfileDialogDelegate
     webContents_.reset(content::WebContents::Create(
         content::WebContents::CreateParams(profile)));
     window.contentView = webContents_->GetNativeView();
+
+    // When a window has layer-backed subviews, its contentView must be
+    // layer-backed or it won't mask the subviews to its rounded corners. See
+    // https://crbug.com/620049
+    // The static_cast is just needed for the 10.10 SDK. It can be removed when
+    // we move to a newer one.
+    static_cast<NSView*>(window.contentView).wantsLayer = YES;
+
     webContentsDelegate_.reset(new UserManagerWebContentsDelegate());
     webContents_->SetDelegate(webContentsDelegate_.get());