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

headless/lib/headless_browser_context_browsertest.cc

Index: headless/lib/headless_browser_context_browsertest.cc
diff --git a/headless/lib/headless_browser_context_browsertest.cc b/headless/lib/headless_browser_context_browsertest.cc
index 1c69aa5b94cacf4bc480064319dc3ac28644c20d..49ab195c09404837850f8c4a70dfd52a2eb2fb1e 100644
--- a/headless/lib/headless_browser_context_browsertest.cc
+++ b/headless/lib/headless_browser_context_browsertest.cc
@@ -8,7 +8,10 @@
 #include "base/memory/ptr_util.h"
 #include "base/strings/stringprintf.h"
 #include "base/threading/thread_restrictions.h"
+#include "content/public/browser/render_view_host.h"
+#include "content/public/browser/web_contents.h"
 #include "content/public/test/browser_test.h"
+#include "headless/lib/browser/headless_web_contents_impl.h"
 #include "headless/public/domains/runtime.h"
 #include "headless/public/headless_browser.h"
 #include "headless/public/headless_browser_context.h"
@@ -267,4 +270,30 @@ IN_PROC_BROWSER_TEST_F(HeadlessBrowserTest, IncognitoMode) {
   EXPECT_TRUE(base::IsDirectoryEmpty(user_data_dir.GetPath()));
 }
 
+IN_PROC_BROWSER_TEST_F(HeadlessBrowserTest, ContextWebPreferences) {
+  // By default, hide_scrollbars should be false.
+  EXPECT_FALSE(WebPreferences().hide_scrollbars);
+
+  // Set hide_scrollbars preference to true for a new BrowserContext.
+  HeadlessBrowserContext* browser_context =
+      browser()
+          ->CreateBrowserContextBuilder()
+          .SetOverrideWebPreferencesCallback(
+              base::Bind([](headless::WebPreferences* preferences) {
+                preferences->hide_scrollbars = true;
+              }))
+          .Build();
+  HeadlessWebContents* web_contents =
+      browser_context->CreateWebContentsBuilder()
+          .SetInitialURL(GURL("about:blank"))
+          .Build();
+
+  // Verify that the preference takes effect.
+  HeadlessWebContentsImpl* contents_impl =
+      HeadlessWebContentsImpl::From(web_contents);
+  EXPECT_TRUE(contents_impl->web_contents()
+                  ->GetRenderViewHost()
+                  ->GetWebkitPreferences().hide_scrollbars);
+}
+
 }  // namespace headless