Make CFX_WideString::FromUTF8() take a CFX_ByteStringC argument.

Methods that take string arguments and do not persist them should
take *StringC types as argument rather than discrete ptr/len args.

Avoids a number of implicit casts from CFX_ByteString to char*.

BUG=

Review URL: https://codereview.chromium.org/1861183002
diff --git a/fpdfsdk/javascript/Document.cpp b/fpdfsdk/javascript/Document.cpp
index 762ba71..75cd2ec 100644
--- a/fpdfsdk/javascript/Document.cpp
+++ b/fpdfsdk/javascript/Document.cpp
@@ -784,8 +784,7 @@
     for (const auto& it : *pDictionary) {
       const CFX_ByteString& bsKey = it.first;
       CPDF_Object* pValueObj = it.second;
-      CFX_WideString wsKey = CFX_WideString::FromUTF8(bsKey, bsKey.GetLength());
-
+      CFX_WideString wsKey = CFX_WideString::FromUTF8(bsKey.AsByteStringC());
       if (pValueObj->IsString() || pValueObj->IsName()) {
         FXJS_PutObjectString(isolate, pObj, wsKey.c_str(),
                              pValueObj->GetUnicodeText().c_str());