[fuzz] Expose Region Op fuzzing to oss-fuzz.

This simplifies the fuzzer (since the important logic
is in FuzzCommon.cpp).

As requested by https://chromium-review.googlesource.com/c/chromium/src/+/2597075

Change-Id: Ia73f88ec584a2147f110755f98542e610100cfff
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/389736
Commit-Queue: Weston Tracey <westont@google.com>
Auto-Submit: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Weston Tracey <westont@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 4008755..525febd 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -3040,6 +3040,14 @@
       ]
     }
 
+    libfuzzer_app("api_regionop") {
+      sources = [
+        "fuzz/FuzzRegionOp.cpp",
+        "fuzz/oss_fuzz/FuzzRegionOp.cpp",
+      ]
+      deps = []
+    }
+
     if (skia_use_gl) {
       libfuzzer_app("api_mock_gpu_canvas") {
         include_dirs = [