Simplify some WriteBlockAtOffset() calls.

These IFX_SeekableStream::WriteBlockAtOffset() calls can be simplified
to just calling WriteBlock(), since they only happen when:

- A stream has just been created, and the offset is 0.
- When the offset is the end, which is what WriteBlock() does.

In turn, WriteBlock() simplifies to WriteString() in most places.

Change-Id: Ifa66932dc9a75787bafba5832e6c100a0a3342bd
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/60890
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

fpdfsdk/fpdfxfa/cpdfxfa_docenvironment.cpp

diff --git a/fpdfsdk/fpdfxfa/cpdfxfa_docenvironment.cpp b/fpdfsdk/fpdfxfa/cpdfxfa_docenvironment.cpp
index 662af58..1e4c836 100644
--- a/fpdfsdk/fpdfxfa/cpdfxfa_docenvironment.cpp
+++ b/fpdfsdk/fpdfxfa/cpdfxfa_docenvironment.cpp
@@ -442,9 +442,7 @@
 
   RetainPtr<IFX_SeekableStream> fileWrite = MakeSeekableStream(pFileHandler);
   if (fileType == FXFA_SAVEAS_XML) {
-    ByteString content = "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\r\n";
-    fileWrite->WriteBlockAtOffset(content.c_str(), fileWrite->GetSize(),
-                                  content.GetLength());
+    fileWrite->WriteString("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\r\n");
     CXFA_FFDoc* ffdoc = m_pContext->GetXFADocView()->GetDoc();
     ffdoc->SavePackage(
         ToNode(ffdoc->GetXFADoc()->GetXFAObject(XFA_HASHCODE_Data)), fileWrite);
@@ -497,13 +495,11 @@
         static const char kFormat[] =
             "\n<pdf href=\"%s\" xmlns=\"http://ns.adobe.com/xdp/pdf/\"/>";
         ByteString content = ByteString::Format(kFormat, bPath.c_str());
-        fileWrite->WriteBlockAtOffset(content.c_str(), fileWrite->GetSize(),
-                                      content.GetLength());
+        fileWrite->WriteString(content.AsStringView());
       }
       auto pAcc = pdfium::MakeRetain<CPDF_StreamAcc>(pStream);
       pAcc->LoadAllDataFiltered();
-      fileWrite->WriteBlockAtOffset(pAcc->GetData(), fileWrite->GetSize(),
-                                    pAcc->GetSize());
+      fileWrite->WriteBlock(pAcc->GetData(), pAcc->GetSize());
     }
   }
   fileWrite->Flush();
@@ -713,10 +709,7 @@
   CXFA_FFDoc* ffdoc = m_pContext->GetXFADocView()->GetDoc();
   RetainPtr<IFX_SeekableStream> fileStream = MakeSeekableStream(pFileHandler);
   if (fileType == FXFA_SAVEAS_XML) {
-    static constexpr char kContent[] =
-        "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\r\n";
-    fileStream->WriteBlockAtOffset(kContent, 0, strlen(kContent));
-
+    fileStream->WriteString("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\r\n");
     ffdoc->SavePackage(
         ToNode(ffdoc->GetXFADoc()->GetXFAObject(XFA_HASHCODE_Data)),
         fileStream);