Avoid static initializers and global variables in 'pdfium_test'.
This CL aims to follow up on thestig's past-land feedback
receveid on https://codereview.chromium.org/2277063003/#msg15
In order to avoid static initializers and global data
declaration, CL adds a data struct named PDFiumTest_FPDF_FORMFILLINFO,
which has FPDF_FORMFILLINFO as the first struct member definition.
This allows the use of reinterpret_cast to/from
PDFiumTest_FPDF_FORMFILLINFO and PDF_FORMFILLINFO.
PDFiumTest_FPDF_FORMFILLINFO also two more struct members,
'loadedPages' and 'formHandle', which were previously global