SkPDF: Fix links to be more valid PDF/A
SkPDF: Fix links to be more valid PDF/A

See:
  https://github.com/veraPDF/veraPDF-validation-profiles/wiki/PDFA-Parts-2-and-3-rules#rule-632-1
  https://github.com/veraPDF/veraPDF-validation-profiles/wiki/PDFA-Parts-2-and-3-rules#rule-632-2

BUG=skia:3010
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1867003004

Review URL: https://codereview.chromium.org/1867003004

rubenvb/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,tmpvar/skia.cc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,tmpvar/skia.cc,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,tmpvar/skia.cc,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,tmpvar/skia.cc,google/skia,google/skia,google/skia,google/skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,tmpvar/skia.cc,aosp-mirror/platform_external_skia,google/skia,rubenvb/skia,HalCanary/skia-hc,tmpvar/skia.cc,rubenvb/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,rubenvb/skia,tmpvar/skia.cc,rubenvb/skia,HalCanary/skia-hc,google/skia,rubenvb/skia,aosp-mirror/platform_external_skia,rubenvb/skia,google/skia,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,tmpvar/skia.cc,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,tmpvar/skia.cc