headless: Allow protocol handler customization

headless/test/headless_browser_test.h

Index: headless/test/headless_browser_test.h
diff --git a/headless/test/headless_browser_test.h b/headless/test/headless_browser_test.h
index 0249cee7198eb0ed57271e7f5430bb30122e3fc3..ca000dcdbf3f8d40ca7644f66294fb440a59a989 100644
--- a/headless/test/headless_browser_test.h
+++ b/headless/test/headless_browser_test.h
@@ -14,6 +14,9 @@ class RunLoop;
 }
 
 namespace headless {
+namespace runtime {
+class EvaluateResult;
+}
 class HeadlessWebContents;
 
 // Base class for tests which require a full instance of the headless browser.
@@ -44,6 +47,11 @@ class HeadlessBrowserTest : public content::BrowserTestBase {
   // Synchronously waits for a tab to finish loading.
   bool WaitForLoad(HeadlessWebContents* web_contents);
 
+  // Synchronously evaluates a script and returns the result.
+  std::unique_ptr<runtime::EvaluateResult> EvaluateScript(
+      HeadlessWebContents* web_contents,
+      const std::string& script);
+
  protected:
   // Returns the browser for the test.
   HeadlessBrowser* browser() const;