[graphite] Make some of the KeyHelper block parameters be optional

Some of the KeyHelper data objects are expensive to default construct and unused w/o a gatherer. Allow those data objects to be optional in the BeginBlock calls.

Bug: b/259547998
Change-Id: Ice9c7b6a70d0386b3d3b61683cb2e48c886e7cbc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/611677
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
5 files changed