Modify big integer library

This patch follows https://pdfium.googlesource.com/pdfium/+/44047c3300d07192a67b1714084cc2d43b1e9bd9

Modify the library to resolve compile error,
add copyright notice and change pdfium.gyp and BUILD.gn

R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/754743003
12 files changed