[a11y] Delegate UTF8ToUTF16 to FML (flutter#31376)

Delegates string encoding conversions between UTF-8 and UTF-16 strings
in //third_party/accessibility/base to our existing implementations in
FML.

Issue: flutter#98061

Author: cbracken

ci/licenses_golden/licenses_flutter

@@ -2109,8 +2109,6 @@ FILE: ../../../flutter/third_party/accessibility/base/simple_token.h
 FILE: ../../../flutter/third_party/accessibility/base/string_utils.cc
 FILE: ../../../flutter/third_party/accessibility/base/string_utils.h
 FILE: ../../../flutter/third_party/accessibility/base/string_utils_unittest.cc
-FILE: ../../../flutter/third_party/accessibility/base/win/string_conversion.cc
-FILE: ../../../flutter/third_party/accessibility/base/win/string_conversion.h
 FILE: ../../../flutter/third_party/accessibility/gfx/transform.cc
 FILE: ../../../flutter/third_party/accessibility/gfx/transform.h
 FILE: ../../../flutter/third_party/tonic/common/build_config.h

third_party/accessibility/base/BUILD.gn

@@ -35,8 +35,6 @@ source_set("base") {
       "win/scoped_safearray.h",
       "win/scoped_variant.cc",
       "win/scoped_variant.h",
-      "win/string_conversion.cc",
-      "win/string_conversion.h",
       "win/variant_util.h",
       "win/variant_vector.cc",
       "win/variant_vector.h",
@@ -52,6 +50,7 @@ source_set("base") {
 
   public_deps = [
     "numerics",
+    "//flutter/fml:string_conversion",
     "//flutter/third_party/accessibility/ax_build",
     "//third_party/dart/runtime/third_party/double-conversion/src:libdouble_conversion",
   ]

third_party/accessibility/base/string_utils.cc

@@ -11,12 +11,9 @@
 #include <regex>
 #include <sstream>
 
+#include "flutter/fml/string_conversion.h"
 #include "third_party/dart/runtime/third_party/double-conversion/src/double-conversion.h"
 
-#if defined(_WIN32)
-#include "base/win/string_conversion.h"
-#endif
-
 #include "no_destructor.h"
 
 namespace base {
@@ -68,21 +65,11 @@ std::u16string ASCIIToUTF16(std::string src) {
 }
 
 std::u16string UTF8ToUTF16(std::string src) {
-#if defined(_WIN32)
-  return WideToUTF16(win::Utf16FromUtf8(src));
-#else
-  std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t> convert;
-  return convert.from_bytes(src);
-#endif
+  return fml::Utf8ToUtf16(src);
 }
 
 std::string UTF16ToUTF8(std::u16string src) {
-#if defined(_WIN32)
-  return win::Utf8FromUtf16(UTF16ToWide(src));
-#else
-  std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t> convert;
-  return convert.to_bytes(src);
-#endif
+  return fml::Utf16ToUtf8(src);
 }
 
 std::u16string WideToUTF16(const std::wstring& src) {