Add a fuzzer for DDL threading

This helps us find any issues with the promise image sharing.

Bug: skia:10286
Change-Id: I393655c2de76f896d9f376765894f84c015b2760
Cq-Include-Trybots: luci.skia.skia.primary:Fuzz-Debian10-Clang
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/374317
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Adlai Holler <adlai@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index b9e4084..78cb5dc 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2276,6 +2276,7 @@
       "fuzz/FuzzCommon.cpp",
       "fuzz/FuzzCommon.h",
       "fuzz/FuzzCreateDDL.cpp",
+      "fuzz/FuzzDDLThreading.cpp",
       "fuzz/FuzzDrawFunctions.cpp",
       "fuzz/FuzzEncoders.cpp",
       "fuzz/FuzzGradients.cpp",