Use the single char overload of append() when possible.

third_party/WebKit/Source/platform/image-decoders/ImageDecoderTestHelpers.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "platform/image-decoders/ImageDecoderTestHelpers.h"
 
 #include "platform/SharedBuffer.h"
 #include "platform/image-decoders/ImageDecoder.h"
 #include "platform/image-decoders/ImageFrame.h"
 #include "platform/testing/UnitTestHelpers.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "wtf/StringHasher.h"
+#include "wtf/text/StringBuilder.h"
 #include <memory>
 
 namespace blink {
 
 PassRefPtr<SharedBuffer> readFile(const char* fileName)
 {
     String filePath = testing::blinkRootDir();
     filePath.append(fileName);
     return testing::readFromFile(filePath);
 }
 
 PassRefPtr<SharedBuffer> readFile(const char* dir, const char* fileName)
 {
-    String filePath = testing::blinkRootDir();
-    filePath.append("/");
+    StringBuilder filePath;
+    filePath.append(testing::blinkRootDir());
+    filePath.append('/');
     filePath.append(dir);
-    filePath.append("/");
+    filePath.append('/');
     filePath.append(fileName);
-
-    return testing::readFromFile(filePath);
+    return testing::readFromFile(filePath.toString());
 }
 
 unsigned hashBitmap(const SkBitmap& bitmap)
 {
     return StringHasher::hashMemory(bitmap.getPixels(), bitmap.getSize());
 }
 
 static unsigned createDecodingBaseline(DecoderCreator createDecoder, SharedBuffer* data)
 {
     std::unique_ptr<ImageDecoder> decoder = createDecoder();
@@ skipping 96 matching lines @@
     // data in a segment, its pointer would no longer be valid.
     segmentedData->data();
 
     ImageFrame* frame = decoder->frameBufferAtIndex(0);
     ASSERT_FALSE(decoder->failed());
     EXPECT_EQ(frame->getStatus(), ImageFrame::FrameComplete);
     EXPECT_EQ(hashBitmap(frame->bitmap()), hash);
 }
 
 } // namespace blink