safe_browsing: Replace a long series of string appends with a single base::StrCat.

This will do a single allocation of the right size.

Bug: None
Change-Id: I777f61ff740f5d375c8aa461431f92dee1fbb6f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2570412
Auto-Submit: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834338}

chrome/browser/safe_browsing/cloud_content_scanning/multipart_uploader.cc

@@ -9,6 +9,7 @@
 #include "base/bind.h"
 #include "base/memory/scoped_refptr.h"
 #include "base/metrics/histogram_functions.h"
+#include "base/strings/strcat.h"
 #include "base/time/time.h"
 #include "components/safe_browsing/core/features.h"
 #include "content/public/browser/browser_task_traits.h"
@@ -75,9 +76,9 @@ void MultipartUploadRequest::Start() {
 std::string MultipartUploadRequest::GenerateRequestBody(
     const std::string& metadata,
     const std::string& data) {
-  return "--" + boundary_ + "\r\n" + kDataContentType + "\r\n\r\n" + metadata +
-         "\r\n--" + boundary_ + "\r\n" + kDataContentType + "\r\n\r\n" + data +
-         "\r\n--" + boundary_ + "--\r\n";
+  return base::StrCat({"--", boundary_, "\r\n", kDataContentType, "\r\n\r\n",
+                       metadata, "\r\n--", boundary_, "\r\n", kDataContentType,
+                       "\r\n\r\n", data, "\r\n--", boundary_, "--\r\n"});
 }
 
 void MultipartUploadRequest::SendRequest() {