introduce a subrun linked list

Create a simple linked list to sequence the subruns preparing
the way for a more focused allocator for managing subruns
at the end of the GrTextBlob.

Change-Id: I595e2ce2810d161332a23405e4615724d2953471
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/366957
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
5 files changed