[base] Prepare //base For String16 Switch

This change prepares //base for the upcoming switch of base::string16 to
std::u16string.

Bug: 911896
Change-Id: I01ca2d0c2634503438f4e9f3b18681d182555b40
Reviewed-on: https://chromium-review.googlesource.com/c/1437195
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628381}

• base/base_paths_win.cc[diff]
• base/bind_internal.h[diff]
• base/command_line.cc[diff]
• base/command_line_unittest.cc[diff]
• base/debug/stack_trace_win.cc[diff]
• base/environment.cc[diff]
• base/environment.h[diff]
• base/environment_unittest.cc[diff]
• base/file_version_info_win.cc[diff]
• base/file_version_info_win.h[diff]
• base/file_version_info_win_unittest.cc[diff]
• base/i18n/file_util_icu.cc[diff]
• base/i18n/file_util_icu_unittest.cc[diff]
• base/i18n/icu_string_conversions_unittest.cc[diff]
• base/i18n/icu_util.cc[diff]
• base/i18n/rtl_unittest.cc[diff]
• base/logging.cc[diff]
• base/logging.h[diff]
• base/memory/platform_shared_memory_region_win.cc[diff]
• base/memory/shared_memory_win.cc[diff]
• base/native_library_win.cc[diff]
• base/power_monitor/power_monitor_device_source_win.cc[diff]
• base/process/launch_win.cc[diff]
• base/process/process_iterator_win.cc[diff]
• base/sampling_heap_profiler/module_cache_win.cc[diff]
• base/strings/string_util_unittest.cc[diff]
• base/system/sys_info_win.cc[diff]
• base/test/test_file_util_win.cc[diff]
• base/test/test_reg_util_win.cc[diff]
• base/test/test_reg_util_win_unittest.cc[diff]
• base/test/test_shortcut_win.cc[diff]
• base/time/time_win_unittest.cc[diff]