Create an interactive site-per-process browsertest class and move DocumentHasFocus there.

content/public/test/browser_test_utils.h

Index: content/public/test/browser_test_utils.h
diff --git a/content/public/test/browser_test_utils.h b/content/public/test/browser_test_utils.h
index a07ed26205bb054dbabac92d274ea4a7bf4134f9..7363dd95fbff8a36aa6d18b77e6a225d9c432319 100644
--- a/content/public/test/browser_test_utils.h
+++ b/content/public/test/browser_test_utils.h
@@ -235,6 +235,10 @@ bool FrameMatchesName(const std::string& name, RenderFrameHost* frame);
 bool FrameIsChildOfMainFrame(RenderFrameHost* frame);
 bool FrameHasSourceUrl(const GURL& url, RenderFrameHost* frame);
 
+// Finds the child frame at the specified |index| for |frame| and returns its
+// RenderFrameHost.  Returns nullptr if such child frame does not exist.
+RenderFrameHost* ChildFrameAt(RenderFrameHost* frame, size_t index);
+
 // Executes the WebUI resource test runner injecting each resource ID in
 // |js_resource_ids| prior to executing the tests.
 //