With --isolate-extensions, forking (via OpenURL) for extensions is not needed.

chrome/browser/extensions/window_open_apitest.cc

Index: chrome/browser/extensions/window_open_apitest.cc
diff --git a/chrome/browser/extensions/window_open_apitest.cc b/chrome/browser/extensions/window_open_apitest.cc
index 249386fa768f3d7597469b6c7c996a95c4d17f45..700e10d26ffe583158893d99a5c298c107d1e644 100644
--- a/chrome/browser/extensions/window_open_apitest.cc
+++ b/chrome/browser/extensions/window_open_apitest.cc
@@ -262,10 +262,10 @@ IN_PROC_BROWSER_TEST_F(ExtensionBrowserTest, WindowOpenInvalidExtension) {
                      url::kStandardSchemeSeparator +
                      last_loaded_extension_id() + "/test.html");
   ui_test_utils::NavigateToURL(browser(), start_url);
-  ASSERT_NO_FATAL_FAILURE(
-      OpenWindow(browser()->tab_strip_model()->GetActiveWebContents(),
+  ASSERT_NO_FATAL_FAILURE(OpenWindow(
+      browser()->tab_strip_model()->GetActiveWebContents(),
       GURL("chrome-extension://thisissurelynotavalidextensionid/newtab.html"),
-      false, NULL));
+      true, NULL));
 
   // If we got to this point, we didn't crash, so we're good.
 }
@@ -283,10 +283,9 @@ IN_PROC_BROWSER_TEST_F(ExtensionBrowserTest, WindowOpenNoPrivileges) {
   ASSERT_NO_FATAL_FAILURE(
       OpenWindow(browser()->tab_strip_model()->GetActiveWebContents(),
                  GURL(std::string(extensions::kExtensionScheme) +
-                     url::kStandardSchemeSeparator +
-                     last_loaded_extension_id() + "/newtab.html"),
-                 false,
-                 &newtab));
+                      url::kStandardSchemeSeparator +
+                      last_loaded_extension_id() + "/newtab.html"),
+                 true, &newtab));
 
   // Extension API should succeed.
   bool result = false;