Headless: expose WebPreferences to embedder, add --hide-scrollbars.

headless/lib/browser/headless_browser_context_impl.cc

Index: headless/lib/browser/headless_browser_context_impl.cc
diff --git a/headless/lib/browser/headless_browser_context_impl.cc b/headless/lib/browser/headless_browser_context_impl.cc
index 375eb13283b401bfacada4bf464ec20f12b77357..df65523f34b7055eb8479088f2b477960588aad7 100644
--- a/headless/lib/browser/headless_browser_context_impl.cc
+++ b/headless/lib/browser/headless_browser_context_impl.cc
@@ -105,7 +105,13 @@ HeadlessBrowserContextImpl::~HeadlessBrowserContextImpl() {
 // static
 HeadlessBrowserContextImpl* HeadlessBrowserContextImpl::From(
     HeadlessBrowserContext* browser_context) {
-  return reinterpret_cast<HeadlessBrowserContextImpl*>(browser_context);
+  return static_cast<HeadlessBrowserContextImpl*>(browser_context);
+}
+
+// static
+HeadlessBrowserContextImpl* HeadlessBrowserContextImpl::From(
+    content::BrowserContext* browser_context) {
+  return static_cast<HeadlessBrowserContextImpl*>(browser_context);
 }
 
 // static
@@ -363,6 +369,13 @@ HeadlessBrowserContext::Builder::EnableUnsafeNetworkAccessWithMojoBindings(
   return *this;
 }
 
+HeadlessBrowserContext::Builder&
+HeadlessBrowserContext::Builder::SetOverrideWebPreferencesCallback(
+    base::Callback<void(WebPreferences*)> callback) {
+  options_->override_web_preferences_callback_ = std::move(callback);
+  return *this;
+}
+
 HeadlessBrowserContext* HeadlessBrowserContext::Builder::Build() {
   if (!mojo_bindings_.empty()) {
     std::unique_ptr<InMemoryProtocolHandler> headless_mojom_protocol_handler(