Revert "Remove CFX_ByteTextBuf from cpdfsdk_interform.cpp and others."

This reverts commit aea80dcc0abc0c310316fa502f91a359bc684758.

Reason for revert: WriteBuf's behavior was altered

Original change's description:
> Remove CFX_ByteTextBuf from cpdfsdk_interform.cpp and others.
> 
> Bug: pdfium:731
> Change-Id: I61d38ab3f2b0ac68b8479ade25bab50f3a73c27b
> Reviewed-on: https://pdfium-review.googlesource.com/9770
> Reviewed-by: Tom Sepez <tsepez@chromium.org>
> Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>

TBR=tsepez@chromium.org,hnakashima@chromium.org,rharrison@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: pdfium:731
Change-Id: Ic66a2641c259173fb3792f7a4206e19ee4707fc9
Reviewed-on: https://pdfium-review.googlesource.com/10150
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
diff --git a/fpdfsdk/javascript/Document.cpp b/fpdfsdk/javascript/Document.cpp
index 77878a5..ba4b2ae 100644
--- a/fpdfsdk/javascript/Document.cpp
+++ b/fpdfsdk/javascript/Document.cpp
@@ -7,7 +7,6 @@
 #include "fpdfsdk/javascript/Document.h"
 
 #include <algorithm>
-#include <sstream>
 #include <utility>
 #include <vector>
 
@@ -383,21 +382,16 @@
       iLength > 4 ? params[4].ToCFXWideString(pRuntime) : L"";
   CFX_WideString cMsg = iLength > 5 ? params[5].ToCFXWideString(pRuntime) : L"";
   CPDFSDK_InterForm* pInterForm = m_pFormFillEnv->GetInterForm();
-  CFX_ByteString sTextBuf = pInterForm->ExportFormToFDFTextBuf();
-  if (sTextBuf.GetLength() == 0)
+  CFX_ByteTextBuf textBuf;
+  if (!pInterForm->ExportFormToFDFTextBuf(textBuf))
     return false;
 
-  FX_STRSIZE nBufSize = sTextBuf.GetLength();
-  char* pMutableBuf = FX_Alloc(char, nBufSize);
-  memcpy(pMutableBuf, sTextBuf.c_str(), nBufSize);
-
   pRuntime->BeginBlock();
   CPDFSDK_FormFillEnvironment* pFormFillEnv = pRuntime->GetFormFillEnv();
-  pFormFillEnv->JS_docmailForm(pMutableBuf, nBufSize, bUI, cTo.c_str(),
-                               cSubject.c_str(), cCc.c_str(), cBcc.c_str(),
-                               cMsg.c_str());
+  pFormFillEnv->JS_docmailForm(textBuf.GetBuffer(), textBuf.GetLength(), bUI,
+                               cTo.c_str(), cSubject.c_str(), cCc.c_str(),
+                               cBcc.c_str(), cMsg.c_str());
   pRuntime->EndBlock();
-  FX_Free(pMutableBuf);
   return true;
 }