Make FPDF_FormHandle be represented as an incomplete type.

Make consistent with other public API types.

Introduce CPDFSDKFormFillEnvironmentFromFPDFFormHandle() and
FPDFFormHandleFromCPDFSDKFormFillEnvironment() helper functions.
Use these to kill off some casts in the process.

Change-Id: I6230ecdb4cecd03076f5e24c8cc49c45ad694da7
Reviewed-on: https://pdfium-review.googlesource.com/39250
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/fpdfsdk/cpdfsdk_helpers.h b/fpdfsdk/cpdfsdk_helpers.h
index 4fd85cf..5de947a 100644
--- a/fpdfsdk/cpdfsdk_helpers.h
+++ b/fpdfsdk/cpdfsdk_helpers.h
@@ -38,6 +38,7 @@
 class CPDF_StructTree;
 class CPDF_TextPage;
 class CPDF_TextPageFind;
+class CPDFSDK_FormFillEnvironment;
 class IPDFSDK_PauseAdapter;
 class FX_PATHPOINT;
 
@@ -197,6 +198,15 @@
   return reinterpret_cast<CPDF_TextPageFind*>(handle);
 }
 
+inline FPDF_FORMHANDLE FPDFFormHandleFromCPDFSDKFormFillEnvironment(
+    CPDFSDK_FormFillEnvironment* handle) {
+  return reinterpret_cast<FPDF_FORMHANDLE>(handle);
+}
+inline CPDFSDK_FormFillEnvironment*
+CPDFSDKFormFillEnvironmentFromFPDFFormHandle(FPDF_FORMHANDLE handle) {
+  return reinterpret_cast<CPDFSDK_FormFillEnvironment*>(handle);
+}
+
 ByteString CFXByteStringFromFPDFWideString(FPDF_WIDESTRING wide_string);
 
 #ifdef PDF_ENABLE_XFA