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>

core/fpdfapi/page/cpdf_image.cpp

diff --git a/core/fpdfapi/page/cpdf_image.cpp b/core/fpdfapi/page/cpdf_image.cpp
index efdd66b..2c3f13e 100644
--- a/core/fpdfapi/page/cpdf_image.cpp
+++ b/core/fpdfapi/page/cpdf_image.cpp
@@ -257,7 +257,7 @@
     int32_t maskWidth = pMaskBitmap->GetWidth();
     int32_t maskHeight = pMaskBitmap->GetHeight();
     std::unique_ptr<uint8_t, FxFreeDeleter> mask_buf;
-    FX_STRSIZE mask_size = 0;
+    int32_t mask_size = 0;
     auto pMaskDict =
         pdfium::MakeUnique<CPDF_Dictionary>(m_pDocument->GetByteStringPool());
     pMaskDict->SetNewFor<CPDF_Name>("Type", "XObject");