Make conversion to CPDF_Dest explicit

Make conversion between CPDF_Dest and its object explicit.

Precursor to taking a second shot at cleaning up the FPDF_*
APIs.  A FPDF_Dest is a CPDF_Array, and a CPDF_Dest
is a structure holding a FPDF_Dest.  This goes against the
convention that FPDF_ types get cast to CPDF_* types, so we
want to make it clear where objects are getting constructed,
etc.

R=thestig@chromium.org

Committed: https://pdfium.googlesource.com/pdfium/+/5a370d77d0db7550e91888f0ff50ee305f0f9f3d

[Tom Sepez, 2015-03-06 01:22:45]

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

[Tom Sepez, 2015-03-06 01:22:46]

Lei, same stuff different class.

[Lei Zhang, 2015-03-06 01:27:47]

lgtm

[Tom Sepez, 2015-03-06 18:34:53]

Committed patchset #1 (id:1) manually as
5a370d77d0db7550e91888f0ff50ee305f0f9f3d (presubmit successful).