Update presubmit/styleguide language around wstring and UTF-16.

wstring is no longer banned in new code, so remove the PRESUBMIT for
that.  Indeed, we do (very rarely) use wstring in cross-platform code
when it only actually matters on Windows.  This should not result in
people wildly misusing wstring, as it is no longer the same type as
string16(/u16string) and thus cannot be silently passed to any APIs.
The only time it's used anymore is generally when it needs to be used,
so this presubmit is just annoying.

Remove outdated language in styleguide around wchar_t and
char16(/char16_t) being the same type.  Instead, provide an explicit
override of the Google styleguide and guidance on how to use UTF-16.

Bug: 911896
Change-Id: Idfce5f43f48ed4a2614c3da0ec793adf91586ff7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2776291


Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#864953}