Simplify reference management in SkPDF

Simplify reference management in SkPDF

Prior to this change, SkPDFObject subclasses were required
to track their resources separately from the document
structure.  (An object has a resource if it depends, via an
indirect reference, on another object).  This led to a lot
of extra code to duplicate effort.  I replace the
getResources() function with the much simpler addResources()
function.  I only define a non-trivial addResources() method
on arrays, dictionaries, and indirect object references.
All other specialized classes simply rely on their parent
class's implementation.

SkPDFObject::addResources() works by recursively walking the
directed graph of object (direct and indirect) references
and adding resources to a set.  It doesn't matter that there
are closed loops in the graph, since we check the set before
walking down a branch.

-  Add SkPDFObject::addResources() virtual function, with
   four implementations
-  Remove SkPDFObject::getResources() virtual function and
   all implementations.
-  Remove SkPDFObject::GetResourcesHelper()
-  Remove SkPDFObject::AddResourceHelper()
-  In SkPDFCatalog::findObjectIndex(), add an object to the
   catalog if it doesn't exist yet.
-  SkPDFCatalog::setSubstitute() no longer sets up resources
-  SkPDFDocument.cpp no longer needs the Streamer object
-  SkPDFDocument.cpp calls fDocCatalog->addResources to build
   the resource list.
-  SkPDFFont::addResource() removed
-  All SkPDF-::fResource sets removed (they are redundant).
-  removed SkPDFImage::addSMask() function
-  SkPDFResourceDict::getReferencedResources() removed.

Motivation: this removes quite a bit of code and makes the
objects slightly slimmer in memory.  Most importantly, this
will lead the way towards removing SkPDFObject's inheritance
from SkRefCnt, which will greatly simplify everything.

Testing: I usually test changes to the PDF backend by
comparing checksums of PDF files rendered from GMs and SKPs
before and after the change.  This change both re-orders and
re-numbers the indirect PDF objects.  I used the qpdf
program to normalize the PDFs and then compared the
normalized outputs from before and after the change; they
matched.

Review URL: https://codereview.chromium.org/870333002