Revert of CQ: Remove libfuzzer trybot from default trybot set. (patchset #1 id:1 of https://codereview.webrtc.org/1764093002/ )

Reason for revert:
libfuzzer has now been moved out of LLVM into third_party/libFuzzer, which we use from https://codereview.webrtc.org/1857673002/

Original issue's description:
> CQ: Remove libfuzzer trybot from default trybot set.
>
> TBR=pbos@webrtc.org
> BUG=chromium:591955
>
> Committed: https://chromium.googlesource.com/external/webrtc/+/2bb708004766ec6acf120b2677bf00810022297d

TBR=pbos@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=chromium:591955
NOTRY=True

Review URL: https://codereview.webrtc.org/1855173002

Cr-Commit-Position: refs/heads/master@{#12214}
diff --git a/infra/config/cq.cfg b/infra/config/cq.cfg
index 9596823..d7bc169 100644
--- a/infra/config/cq.cfg
+++ b/infra/config/cq.cfg
@@ -46,8 +46,7 @@
       builders { name: "linux_compile_dbg" }
       builders { name: "linux_gn_dbg" }
       builders { name: "linux_gn_rel" }
-      # Disabled for now. See http://crbug.com/591955 for details.
-      #builders { name: "linux_libfuzzer_rel" }
+      builders { name: "linux_libfuzzer_rel" }
       builders { name: "linux_msan" }
       builders { name: "linux_rel" }
       builders { name: "linux_tsan2" }