[headless] Make --hide-scrollbars affect all BrowserContexts.

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 047f408a0f4762f35cac78e89c8542603f8bdf71..cd05f9796cec82027c032dfc5a6fdf020d309db9 100644
--- a/headless/app/headless_shell.cc
+++ b/headless/app/headless_shell.cc
@@ -92,13 +92,6 @@ class HeadlessShell : public HeadlessWebContents::Observer,
 
       context_builder.SetProtocolHandlers(std::move(protocol_handlers));
     }
-    if (base::CommandLine::ForCurrentProcess()->HasSwitch(
-            switches::kHideScrollbars)) {
-      context_builder.SetOverrideWebPreferencesCallback(
-          base::Bind([](WebPreferences* preferences) {
-            preferences->hide_scrollbars = true;
-          }));
-    }
     browser_context_ = context_builder.Build();
     browser_->SetDefaultBrowserContext(browser_context_);
 
@@ -518,6 +511,11 @@ int HeadlessShellMain(int argc, const char** argv) {
     builder.SetWindowSize(parsed_window_size);
   }
 
+  if (command_line.HasSwitch(switches::kHideScrollbars)) {
+    builder.SetOverrideWebPreferencesCallback(base::Bind([](
+        WebPreferences* preferences) { preferences->hide_scrollbars = true; }));
+  }
+
   return HeadlessBrowserMain(
       builder.Build(),
       base::Bind(&HeadlessShell::OnStart, base::Unretained(&shell)));