Prevent base::CheckedNumeric from leaking outside of pdfium.

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

Committed: https://pdfium.googlesource.com/pdfium/+/62120b79bbaca7430a8fc7ba0846b0a84f40542d

[Tom Sepez, 2015-02-03 23:47:02]

tsepez@chromium.org changed reviewers:
+ jam@chromium.org

[Tom Sepez, 2015-02-03 23:47:03]

John, please review.  I happened to notice this while putting together the
unrelated ill-fated mini_chromium CL, so at least something will come from that
effort :).

[jam, 2015-02-04 16:37:41]

lgtm

[Tom Sepez, 2015-02-04 18:37:22]

Committed patchset #2 (id:20001) manually as
62120b79bbaca7430a8fc7ba0846b0a84f40542d (presubmit successful).