Use Swiftshader by default when fuzzing

Note: This still requires Chromium to have set dawn_swiftshader_dir
to take effect.

Bug: dawn:295
Change-Id: I0c953196630dd2694f68d290e6f3f1e427c20a2f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/14821
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
diff --git a/scripts/dawn_features.gni b/scripts/dawn_features.gni
index 7cbb82f..f2e9586 100644
--- a/scripts/dawn_features.gni
+++ b/scripts/dawn_features.gni
@@ -56,7 +56,8 @@
   # dawn_swiftshader_dir variable in build_overrides/dawn.gni
   # Because of how the Vulkan loader works, setting this make Dawn only able
   # to find the Swiftshader ICD and not the others.
-  dawn_use_swiftshader = false
+  # Enabled by default when fuzzing.
+  dawn_use_swiftshader = build_with_chromium && use_fuzzing_engine
 
   # Enables error injection for faking failures to native API calls
   dawn_enable_error_injection =