Fix recursion bug in blocking a fetch and add a test

headless/public/util/testing/generic_url_request_mocks.cc

Index: headless/public/util/testing/generic_url_request_mocks.cc
diff --git a/headless/public/util/testing/generic_url_request_mocks.cc b/headless/public/util/testing/generic_url_request_mocks.cc
index b23796c597734c817449ce1602ae0ae35896ea15..4060950e122d886af2c6710d5145a881271251bf 100644
--- a/headless/public/util/testing/generic_url_request_mocks.cc
+++ b/headless/public/util/testing/generic_url_request_mocks.cc
@@ -13,14 +13,17 @@ class URLRequestJob;
 namespace headless {
 
 // MockGenericURLRequestJobDelegate
-MockGenericURLRequestJobDelegate::MockGenericURLRequestJobDelegate() {}
+MockGenericURLRequestJobDelegate::MockGenericURLRequestJobDelegate()
+    : should_block_(false) {}
 MockGenericURLRequestJobDelegate::~MockGenericURLRequestJobDelegate() {}
 
 bool MockGenericURLRequestJobDelegate::BlockOrRewriteRequest(
     const GURL& url,
     const std::string& referrer,
     GenericURLRequestJob::RewriteCallback callback) {
-  return false;
+  if (should_block_)
+    callback(GenericURLRequestJob::RewriteResult::kDeny, GURL());
+  return should_block_;
 }
 
 const GenericURLRequestJob::HttpResponse*