Fix HeadlessWebContentsTest.BrowserOpenInTab.

As of https://crrev.com/617708 the expectation is that all renderer
initiated navigations must specify the origin of the document that
started the navigation.

This CL adds support for initiator origin in headless code.

Bug: 882053
Change-Id: I67dfff88932174f8098f4349412c37fac6e54381
Reviewed-on: https://chromium-review.googlesource.com/c/1395064Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Nasko Oskov <nasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619831}

headless/lib/browser/headless_web_contents_impl.cc

@@ -153,6 +153,7 @@ class HeadlessWebContentsImpl::Delegate : public content::WebContentsDelegate {
     }
 
     content::NavigationController::LoadURLParams load_url_params(params.url);
+    load_url_params.initiator_origin = params.initiator_origin;
     load_url_params.source_site_instance = params.source_site_instance;
     load_url_params.transition_type = params.transition;
     load_url_params.frame_tree_node_id = params.frame_tree_node_id;