Use ptr diff to encode function and padding.

Use ptr diff to encode function and padding.

This drops allocation overhead from 16 bytes down to 4 bytes.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5066

CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Win8-MSVC-ShuttleB-GPU-HD4600-x86_64-Debug-Trybot;master.client.skia:Test-Ubuntu-Clang-GCE-CPU-AVX2-x86_64-Debug-GN-Trybot

Change-Id: Id4665fd914dfb679996cefaf36fce6ef225fafb5
Reviewed-on: https://skia-review.googlesource.com/5066
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Herb Derby <herb@google.com>