Fix the noisiest variable shadowing warnings in pdfium.

Three functions in fx_coordinates.h account for 60% of the warnings
when building with VS 2015, due to variable shadowing. Renaming the
function parameters is safe, resolves the warnings, and reduces
confusion.

R=tsepez@chromium.org
BUG=440500

Review URL: https://codereview.chromium.org/1077083003