[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>
4 files changed