Update buffer safety comments in fx_extension.cpp.

Header is already specifying UNSAFE_BUFFER_USAGE for these
two functions.

Bug: 42271176
Change-Id: Icae84af6b28a7e26db55048e72d1c4e0d60c83a0
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/120591
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Thomas Sepez <tsepez@google.com>
Commit-Queue: Tom Sepez <tsepez@chromium.org>

core/fxcrt/fx_extension.cpp

diff --git a/core/fxcrt/fx_extension.cpp b/core/fxcrt/fx_extension.cpp
index f4fa200..67a5660 100644
--- a/core/fxcrt/fx_extension.cpp
+++ b/core/fxcrt/fx_extension.cpp
@@ -30,7 +30,8 @@
 }  // namespace
 
 float FXSYS_wcstof(const wchar_t* pwsStr, size_t nLength, size_t* pUsedLen) {
-  auto copied = UNSAFE_TODO(WideString::Create(pwsStr, nLength));
+  // SAFETY: required from caller, enforced by UNSAFE_BUFFER_USAGE in header.
+  auto copied = UNSAFE_BUFFERS(WideString::Create(pwsStr, nLength));
   wchar_t* endptr = nullptr;
   float result = wcstof(copied.c_str(), &endptr);
   if (result != result) {
@@ -42,13 +43,12 @@
   return result;
 }
 
-// TODO(tsepez): should be UNSAFE_BUFFER_USAGE.
 wchar_t* FXSYS_wcsncpy(wchar_t* dstStr, const wchar_t* srcStr, size_t count) {
   DCHECK(dstStr);
   DCHECK(srcStr);
   DCHECK(count > 0);
 
-  // SAFETY: required from caller.
+  // SAFETY: required from caller, enforced by UNSAFE_BUFFER_USAGE in header.
   UNSAFE_BUFFERS({
     for (size_t i = 0; i < count; ++i) {
       dstStr[i] = srcStr[i];