Remove use of GetBuffer()/ReleaseBuffer() when c_str() is sufficient.

Greatly simplify the code which was neither changing allocations nor
doing any actual work. Eventually GetBuffer() will return a span, and
this makes fewer places to change.

Comment AsFPDFWideString() as a place where we may want a redundant
copy, and make available outside the .cpp file.

Fix one "const" along the way.

Change-Id: I187758b69a0ba3501b3622f6c06280793cd5d464
Reviewed-on: https://pdfium-review.googlesource.com/30011
Reviewed-by: dsinclair <dsinclair@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/fpdfsdk/cpdfsdk_interform.cpp b/fpdfsdk/cpdfsdk_interform.cpp
index 6d3e9cd..e83e599 100644
--- a/fpdfsdk/cpdfsdk_interform.cpp
+++ b/fpdfsdk/cpdfsdk_interform.cpp
@@ -521,12 +521,7 @@
     ByteString csBValue = pField->GetStringFor("V");
     WideString csWValue = PDF_DecodeText(csBValue);
     ByteString csValue_b = ByteString::FromUnicode(csWValue);
-
-    fdfEncodedData << name_b.GetBuffer(name_b.GetLength());
-    name_b.ReleaseBuffer(name_b.GetStringLength());
-    fdfEncodedData << "=";
-    fdfEncodedData << csValue_b.GetBuffer(csValue_b.GetLength());
-    csValue_b.ReleaseBuffer(csValue_b.GetStringLength());
+    fdfEncodedData << name_b.c_str() << "=" << csValue_b.c_str();
     if (i != pFields->GetCount() - 1)
       fdfEncodedData << "&";
   }