Move iframe@seamless behind the "Experimental WebKit Features" flag.

content/renderer/render_thread_impl.cc

Index: content/renderer/render_thread_impl.cc
diff --git a/content/renderer/render_thread_impl.cc b/content/renderer/render_thread_impl.cc
index 9b9adda6f57b5cee2f60dde8931a61fdbe26e3be..ff6c8cb5a489bf26ca9aa4300f7cc131144ceb98 100644
--- a/content/renderer/render_thread_impl.cc
+++ b/content/renderer/render_thread_impl.cc
@@ -698,6 +698,9 @@ void RenderThreadImpl::EnsureWebKitInitialized() {
   WebRuntimeFeatures::enableCSSRegions(
       command_line.HasSwitch(switches::kEnableExperimentalWebKitFeatures));
 
+  WebRuntimeFeatures::enableSeamlessIFrames(
+      command_line.HasSwitch(switches::kEnableExperimentalWebKitFeatures));
+
   FOR_EACH_OBSERVER(RenderProcessObserver, observers_, WebKitInitialized());
 
   devtools_agent_message_filter_ = new DevToolsAgentFilter();