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 <jdoerrie@chromium.org>
> Reviewed-by: Matt Menke <mmenke@chromium.org>
> Reviewed-by: Victor Costan <pwnall@chromium.org>
> Reviewed-by: Will Harris <wfh@chromium.org>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Auto-Submit: Jan Wilken Dörrie <jdoerrie@chromium.org>
> 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 <jdoerrie@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790444}

• base/i18n/file_util_icu_unittest.cc[diff]
• base/i18n/rtl_unittest.cc[diff]
• base/strings/string_util_win.h[diff]
• content/public/test/url_loader_interceptor.cc[diff]
• net/base/filename_util.cc[diff]
• net/base/filename_util_internal.cc[diff]
• net/base/filename_util_unittest.cc[diff]
• net/base/mime_util.cc[diff]
• net/base/network_interfaces_unittest.cc[diff]
• net/base/platform_mime_util_win.cc[diff]
• net/disk_cache/backend_unittest.cc[diff]
• net/disk_cache/blockfile/file_win.cc[diff]
• net/disk_cache/cache_util.cc[diff]
• net/disk_cache/cache_util_win.cc[diff]
• net/disk_cache/simple/simple_util_win.cc[diff]
• net/dns/dns_config_service_win.cc[diff]
• net/dns/dns_config_service_win.h[diff]
• net/dns/dns_config_service_win_unittest.cc[diff]
• net/proxy_resolution/win/proxy_config_service_win.cc[diff]
• net/proxy_resolution/win/proxy_config_service_win.h[diff]
• net/quic/platform/impl/quic_default_proof_providers_impl.cc[diff]
• net/quic/platform/impl/quic_flags_impl.cc[diff]
• net/test/embedded_test_server/request_handler_util.cc[diff]
• net/test/embedded_test_server/request_handler_util.h[diff]
• net/test/spawned_test_server/base_test_server.cc[diff]
• sandbox/win/src/sandbox_policy_diagnostic.cc[diff]
• sql/database.cc[diff]
• sql/statement.cc[diff]
• sql/statement.h[diff]