Convert int* references to FX_STRSIZE

Through out the code base there are numerous places where variables
are declared using a signed integer type when interacting with the
string classes, since they assume that FX_STRSIZE is 'int'. As part of
changing the underling type of FX_STRSIZE to be unsigned, these
locations are being changed to use FX_STRSIZE. This is necessary as
part of converting the type, but has been broken off into a separate CL,
since it should be low risk.

Some related cleanups that are low risk are included as part of
this CL.

BUG=pdfium:828

Change-Id: Ifaae54ad195ccde0fe8672f71271d29a6ebd65fd
Reviewed-on: https://pdfium-review.googlesource.com/12210
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
diff --git a/fpdfsdk/javascript/Document.cpp b/fpdfsdk/javascript/Document.cpp
index 9b82147..d8934a4 100644
--- a/fpdfsdk/javascript/Document.cpp
+++ b/fpdfsdk/javascript/Document.cpp
@@ -1016,7 +1016,7 @@
     return false;
   }
   CFX_WideString wsFilePath = m_pFormFillEnv->JS_docGetFilePath();
-  int32_t i = wsFilePath.GetLength() - 1;
+  FX_STRSIZE i = wsFilePath.GetLength() - 1;
   for (; i >= 0; i--) {
     if (wsFilePath[i] == L'\\' || wsFilePath[i] == L'/')
       break;