kill IPDF_DocParser().

Its fine to program to interfaces, but since the sole concrete implementation
is in the same header as the interface, the code is bypassing it anyways. We
can de-virtualize some things along the way, and remove two non-existent
function prototypes from one of the headers.

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1158053003