[fuzz] Add libfuzzer binaries to BUILD.gn

For the longest time, the libfuzzer binaries used by oss-fuzz
were just hacked onto the BUILD.gn file. This removes that patch
and makes them buildable from Skia proper.

After this, there should not be any modifications oss-fuzz needs
to do to a Skia checkout before it builds and runs.

Of note, oss-fuzz will define skia_use_libfuzzer_defaults to be
false so it can control those flags with more finesse (e.g.
fuzz with ASAN, fuzz with hong fuzz instead of libfuzzer). I
added on skia_use_libfuzzer_defaults so that a normal developer
gets something that works by default.

Bug: skia:10713
Change-Id: I024f5805060cba8f8560e1c2569b9309fb49a564
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/316536
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
3 files changed