SkPDF: add drop() virtual to release resources early.

SkPDF: add drop() virtual to release resources early.

Call drop() after calling emitObject() on top-level objects.  In Debug
mode, assert that each object is emited exactly once by asserting that
emitObject is never called after drop().  Same for addResources().

To make sure that top level objects don't get deleted prematurely,
SkPDFObjNumMap takes a reference.

Motivation: save RAM.  Allow even earlier serialization with later
changes.

Also: Switch some SkTDArrays to SkTArrays.

BUG=skia:5087
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1790023003

Review URL: https://codereview.chromium.org/1790023003