Simplify OnReadCompleted() logic that appends bytes to the client buffer

ios/net/crn_http_protocol_handler.mm

 // Copyright 2012 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.
 
 #import "ios/net/crn_http_protocol_handler.h"
 
 #include <stdint.h>
 
 #include <memory>
 #include <utility>
@@ skipping 516 matching lines @@
   uint64_t total_bytes_read = 0;
   while (bytes_read > 0) {
     base::scoped_nsobject<NSMutableData> data(
         [[NSMutableData alloc] initWithCapacity:bytes_read]);
     // |bytes_read| should always be less or equal to |kClientMaxBufferSize|.
     // This is ensured by the fact that the max read buffer size (i.e.
     // |kIOBufferSize|) is always smaller or equal to |kClientMaxBufferSize|.
     while (bytes_read > 0 &&
            [data length] + bytes_read <= kClientMaxBufferSize) {
       total_bytes_read += bytes_read;
-      NSUInteger data_length = [data length];
-      [data increaseLengthBy:bytes_read];
-      memcpy(reinterpret_cast<char*>([data mutableBytes]) + data_length,
-             buffer_->data(), bytes_read);
+      [data appendBytes:buffer_->data() length:bytes_read];
       bytes_read = request->Read(buffer_.get(), kIOBufferSize);
     }
 
     if ([data length] > 0) {
       // If the data is not encoded in UTF8, the NSString is nil.
       DVLOG(3) << "To client:" << std::endl
                << base::SysNSStringToUTF8([[NSString alloc]
                       initWithData:data
                           encoding:NSUTF8StringEncoding]);
       [client_ didLoadData:data];
@@ skipping 416 matching lines @@
                  base::Bind(&net::HttpProtocolHandlerCore::Cancel, _core));
   [_protocolProxy invalidate];
 }
 
 - (void)stopLoading {
   [self cancelRequest];
   _protocolProxy.reset();
 }
 
 @end