headless: Hide scrollbar from screenshots.

headless/app/headless_shell.cc

Index: headless/app/headless_shell.cc
diff --git a/headless/app/headless_shell.cc b/headless/app/headless_shell.cc
index f8145054054325e9a5ccaa029622485a36f86b0f..6cea6ad8cc28ef256e3fa0dfc78587a82295731f 100644
--- a/headless/app/headless_shell.cc
+++ b/headless/app/headless_shell.cc
@@ -336,6 +336,11 @@ int main(int argc, const char** argv) {
         command_line.GetSwitchValueASCII(switches::kHostResolverRules));
   }
 
+  if (!command_line.HasSwitch(switches::kDisableOverlayScrollbar)) {
+    // Enable overlay scrollbar by default to hide it from screenshots.
+    command_line.AppendSwitch(switches::kEnableOverlayScrollbar);
+  }
+
   return HeadlessBrowserMain(
       builder.Build(),
       base::Bind(&HeadlessShell::OnStart, base::Unretained(&shell)));