src/pdf: code cleanup

src/pdf: code cleanup

  * SkPDFCanon: remove unnecessary abstraction
  * Make use of SkTHashMap<K, sk_sp<T>>.
  * Remove unncessary struct constructors.
  * More factory fns return sk_sp<T>
  * SkPDFUtility::GetCachedT<T> factored out.

Change-Id: I4055a131b43fe2588fd042b769cd09fff8a3466c
Reviewed-on: https://skia-review.googlesource.com/13655
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>