testing/libfuzzer: build individual_fuzztest_wrapper only in chromium

This is to fix `gn gen` failure in the testing roll CL to dawn.
e.g.
https://logs.chromium.org/logs/dawn/buildbucket/cr-buildbucket/8745995919527078337/+/u/gn_gen__2_/stdout

Bug: 341220050
Change-Id: If4525ed12b75a71d38656060aa2a8df3d55450ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5601256


Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Adrian Taylor <adetaylor@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1311728}