Add the ability to override WebContentsBindingSet binders for testing

content/test/BUILD.gn

Index: content/test/BUILD.gn
diff --git a/content/test/BUILD.gn b/content/test/BUILD.gn
index ebe2facce815eda6ee9fe082f086a743b4f85ea0..bff61e7845c1694a5ab24dc41fc667f442567c6b 100644
--- a/content/test/BUILD.gn
+++ b/content/test/BUILD.gn
@@ -139,6 +139,7 @@ static_library("test_support") {
     "../public/test/text_input_test_utils_mac.mm",
     "../public/test/unittest_test_suite.cc",
     "../public/test/unittest_test_suite.h",
+    "../public/test/web_contents_binding_set_test_binder.h",
     "../public/test/web_contents_tester.cc",
     "../public/test/web_contents_tester.h",
     "accessibility_browser_test_utils.cc",
@@ -662,6 +663,7 @@ test("content_browsertests") {
     "../browser/web_contents/opened_by_dom_browsertest.cc",
     "../browser/web_contents/web_contents_impl_browsertest.cc",
     "../browser/web_contents/web_contents_view_aura_browsertest.cc",
+    "../browser/web_contents_binding_set_browsertest.cc",
     "../browser/webkit_browsertest.cc",
     "../browser/webui/web_ui_mojo_browsertest.cc",
     "../browser/zoom_browsertest.cc",