Reland "[base] Prepare //net for base::string16 switch"

This is a reland of ee99ff7f2e5b8a5d90a1c5fd3fe2e3eb5a8fb8f6

Original change's description:
> [base] Prepare //net for base::string16 switch
>
> This change prepares //net for the switch of base::string16 to
> std::u16string on all platforms. This was achieved by making
> net_unittests compile on Windows after the switch was made.
>
> Furthermore, it fixes small issues in //base and //sql.
>
> Bug: 911896
> Change-Id: I11fd80998e9f5e954a87870533685a3f77fa42d9
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303431
> Commit-Queue: Jan Wilken Dörrie <[email protected]>
> Reviewed-by: Matt Menke <[email protected]>
> Reviewed-by: Victor Costan <[email protected]>
> Reviewed-by: Will Harris <[email protected]>
> Reviewed-by: Daniel Cheng <[email protected]>
> Auto-Submit: Jan Wilken Dörrie <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#790126}

TBR=wfh

Bug: 911896
Change-Id: Ibbd6cb6695b40149971186f05ffb6b859acd68f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2309689
Commit-Queue: Jan Wilken Dörrie <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Reviewed-by: Matt Menke <[email protected]>
Reviewed-by: Victor Costan <[email protected]>
Cr-Commit-Position: refs/heads/master@{#790444}
diff --git a/net/base/filename_util_unittest.cc b/net/base/filename_util_unittest.cc
index 9ad798c..8e3b6602 100644
--- a/net/base/filename_util_unittest.cc
+++ b/net/base/filename_util_unittest.cc
@@ -39,14 +39,14 @@
 // std::u16string.
 std::wstring FilePathAsWString(const base::FilePath& path) {
 #if defined(OS_WIN)
-  return base::UTF16ToWide(path.value());
+  return path.value();
 #elif defined(OS_POSIX) || defined(OS_FUCHSIA)
   return base::UTF8ToWide(path.value());
 #endif
 }
 base::FilePath WStringAsFilePath(const std::wstring& str) {
 #if defined(OS_WIN)
-  return base::FilePath(base::WideToUTF16(str));
+  return base::FilePath(str);
 #elif defined(OS_POSIX) || defined(OS_FUCHSIA)
   return base::FilePath(base::WideToUTF8(str));
 #endif