Use StringView for String::append and ::insert.

third_party/WebKit/Source/modules/fetch/DataConsumerHandleTestUtil.h

Index: third_party/WebKit/Source/modules/fetch/DataConsumerHandleTestUtil.h
diff --git a/third_party/WebKit/Source/modules/fetch/DataConsumerHandleTestUtil.h b/third_party/WebKit/Source/modules/fetch/DataConsumerHandleTestUtil.h
index 2506af4e00842c49be6d5ccd8afc48e7940f5f76..52986bec48af7ae0352f5c2eab867e492227ea1d 100644
--- a/third_party/WebKit/Source/modules/fetch/DataConsumerHandleTestUtil.h
+++ b/third_party/WebKit/Source/modules/fetch/DataConsumerHandleTestUtil.h
@@ -88,18 +88,26 @@ public:
             void recordAttach(const String& handle)
             {
                 MutexLocker locker(m_loggingMutex);
-                m_result.append("A reader is attached to " + handle + " on " + currentThreadName() + ".\n");
+                m_result.append("A reader is attached to ");
+                m_result.append(handle);
+                m_result.append(" on ");
+                m_result.append(currentThreadName());
+                m_result.append(".\n");
             }
             void recordDetach(const String& handle)
             {
                 MutexLocker locker(m_loggingMutex);
-                m_result.append("A reader is detached from " + handle + " on " + currentThreadName() + ".\n");
+                m_result.append("A reader is detached from ");
+                m_result.append(handle);
+                m_result.append(" on ");
+                m_result.append(currentThreadName());
+                m_result.append(".\n");
             }
 
-            const String& result()
+            String result()
             {
                 MutexLocker locker(m_loggingMutex);
-                return m_result;
+                return m_result.toString();
             }
 
             void registerThreadHolder(ThreadHolder* holder)
@@ -144,7 +152,7 @@ public:
 
             // Protects |m_result|.
             Mutex m_loggingMutex;
-            String m_result;
+            StringBuilder m_result;
 
             // Protects |m_holder|.
             Mutex m_holderMutex;
@@ -216,7 +224,7 @@ public:
 
         void resetReader() { m_reader = nullptr; }
         void signalDone() { m_waitableEvent->signal(); }
-        const String& result() { return m_context->result(); }
+        String result() { return m_context->result(); }
         void postTaskToReadingThread(const WebTraceLocation& location, std::unique_ptr<CrossThreadClosure> task)
         {
             m_context->postTaskToReadingThread(location,  std::move(task));