Extracting and unittesting PrepareDropDataForChildProcess function.

content/test/BUILD.gn

Index: content/test/BUILD.gn
diff --git a/content/test/BUILD.gn b/content/test/BUILD.gn
index 6338a696ffb50cd940375faaff73ff4ebfbc848c..364d3e41623645cc066b2bafa2477d117999b420 100644
--- a/content/test/BUILD.gn
+++ b/content/test/BUILD.gn
@@ -929,8 +929,8 @@ test("content_browsertests") {
       "../browser/webrtc/webrtc_webcam_browsertest.h",
     ]
     deps += [
-      "//testing/perf",
       "//content/public/common:features",
+      "//testing/perf",
     ]
   }
 
@@ -1085,6 +1085,7 @@ test("content_unittests") {
     "../browser/download/parallel_download_utils_unittest.cc",
     "../browser/download/rate_estimator_unittest.cc",
     "../browser/download/save_package_unittest.cc",
+    "../browser/fileapi/browser_file_system_helper_unittest.cc",
     "../browser/fileapi/file_system_operation_runner_unittest.cc",
     "../browser/fileapi/fileapi_message_filter_unittest.cc",
     "../browser/fileapi/upload_file_system_file_element_reader_unittest.cc",