Fix GCC build
Fix GCC build

-Wno-sign-compare also for GCC.

-Wno-error=strict-overflow to ignore the following warning.

In file included from
../../third_party/pdfium/core/include/fpdfapi/cpdf_object.h:10:0,
                 from
../../third_party/pdfium/core/include/fpdfapi/cpdf_stream.h:10,
                 from
../../third_party/pdfium/core/include/fpdfapi/fpdf_resource.h:12,
                 from
../../third_party/pdfium/core/fpdfapi/fpdf_font/font_int.h:13,
                 from
../../third_party/pdfium/core/fpdfapi/fpdf_font/fpdf_font_cid.cpp:7:
../../third_party/pdfium/core/include/fxcrt/fx_string.h: In function
'CFX_ByteString {anonymous}::CMap_GetString(const CFX_ByteStringC&)':
../../third_party/pdfium/core/include/fxcrt/fx_string.h:111:19: warning:
assuming signed overflow does not occur when assuming that (X - c) > X is always
false [-Wstrict-overflow]
     if (count < 0 || count > m_Length - index) {

BUG=589724
TEST=build with GCC
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1785943002 .

DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium