Use IPDF_DataAvail interface to hide CPDF_DataAvail.

Currently, no callers go through the Interface, which makes having a
separate interface class kind of pointless.  After converting callers
away from using the CPDF_DataAvail concrete class, it can be moved
from the header to the .cpp file.

R=bo_xu@foxitsoftware.com

Review URL: https://codereview.chromium.org/873523002
3 files changed