Prevent base::CheckedNumeric from leaking outside of pdfium.

Headers in /include directories should be free of implementation details
from third_party.  Put the types into a new header outside of /include.

Requires https://codereview.chromium.org/902443003/ before a version containing this patch is rolled into chromium.

R=jam@chromium.org

Review URL: https://codereview.chromium.org/896023003
13 files changed