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

headless/public/headless_browser_context.h

Index: headless/public/headless_browser_context.h
diff --git a/headless/public/headless_browser_context.h b/headless/public/headless_browser_context.h
index d041c0a3e923056a99c5421e013ee2730a40ffda..b6d41576621610ecf4c55451e7bc1f5600a4bed4 100644
--- a/headless/public/headless_browser_context.h
+++ b/headless/public/headless_browser_context.h
@@ -12,6 +12,7 @@
 #include <vector>
 
 #include "base/optional.h"
+#include "content/public/common/web_preferences.h"
 #include "headless/public/headless_export.h"
 #include "headless/public/headless_web_contents.h"
 #include "net/base/host_port_pair.h"
@@ -25,6 +26,10 @@ namespace headless {
 class HeadlessBrowserImpl;
 class HeadlessBrowserContextOptions;
 
+// Imported into headless namespace for
+// Builder::SetOverrideWebPreferencesCallback().
+using content::WebPreferences;
+
 using ProtocolHandlerMap = std::unordered_map<
     std::string,
     std::unique_ptr<net::URLRequestJobFactory::ProtocolHandler>>;
@@ -97,6 +102,14 @@ class HEADLESS_EXPORT HeadlessBrowserContext::Builder {
   Builder& EnableUnsafeNetworkAccessWithMojoBindings(
       bool enable_http_and_https_if_mojo_used);
 
+  // Set a callback that is invoked to override WebPreferences for RenderViews
+  // created within this HeadlessBrowserContext.

[Sami, 2016/09/29 09:38:26]

This is called on the browser main thread, right? And possibly multiple times?

[Eric Seckler, 2016/09/30 09:17:50]

It's definitely called multiple times (whenever the prefs of a RenderView
change). It looks like OverrideWebkitPrefs is executed on the browser main
thread and you don't have to worry about it being called by different threads,
but I also can't find anything explicitly confirming this.

Added a comment to these effects.

+  //
+  // WARNING: We cannot provide any guarantees about the stability of the
+  // exposed WebPreferences API, so use with care.
+  Builder& SetOverrideWebPreferencesCallback(
+      std::function<void(WebPreferences*)> callback);

[Sami, 2016/09/29 09:38:26]

This should probably be a base::Callback since most of Chrome is still built
that way.

[Eric Seckler, 2016/09/30 09:17:50]

Done.

+
   // By default |HeadlessBrowserContext| inherits the following options from
   // the browser instance. The methods below can be used to override these
   // settings. See HeadlessBrowser::Options for their meaning.