[DawnLPM] Prevent protobuf generation on non-fuzzing builds

`proto_library` will generate protobuf files in official builds
even if it's only a dependency of an action that is only built during fuzzing builds. Fuzzer breakages should only break chromium.fuzz which is monitored for such breakages, not the entire tree.

Bug: chromium:1455317
Change-Id: I9aee8d666ba745a08da7d9df812c72793ac7bdce
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/152562
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Brendon Tiszka <tiszka@chromium.org>
diff --git a/src/dawn/fuzzers/BUILD.gn b/src/dawn/fuzzers/BUILD.gn
index 4fd3f3c..a3686c9 100644
--- a/src/dawn/fuzzers/BUILD.gn
+++ b/src/dawn/fuzzers/BUILD.gn
@@ -113,7 +113,8 @@
   additional_configs = [ "${dawn_root}/src/dawn/common:internal_config" ]
 }
 
-if (is_dawn_lpm_fuzzer && build_with_chromium && dawn_use_swiftshader) {
+if (is_dawn_lpm_fuzzer && build_with_chromium && dawn_use_swiftshader &&
+    !disable_libfuzzer && use_fuzzing_engine) {
   import("//third_party/protobuf/proto_library.gni")
   import("${dawn_root}/generator/dawn_generator.gni")