[base] Prepare CrashReporterClient for string16 switch

This change prepares CrashReporterClient for the upcoming base::string16
switch on Windows. It performs the following mechanical changes:

* Replace base::string16 with std::wstring
* Replace #include "base/strings/string16.h" with #include <string>
* Replace STRING16_LITERAL("...") with L"..."
* Replace UTF16 conversion functions with their wide equivalents,
  removing redundant calls to base::WideToUTF16
* Adding base::AsWString(...) when interfacing with x-platform APIs

Bug: 911896
Change-Id: Iecf67466a21d0f53ba13cdfdb2a82ee69f4f70f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2673415
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#851776}