Disable the sources assignment filter for platform specific files.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4918

Change-Id: Ie9e0db55eae5ad7cad82db5270a8bb6e80d3b3bb
Reviewed-on: https://skia-review.googlesource.com/4918
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 7b627f1..f765cc4 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -545,6 +545,11 @@
     ":xml",
   ]
 
+  # This file (and all GN files in Skia) are designed to work with an
+  # empty sources assignment filter; we handle all that explicitly.
+  # We clear the filter here for clients who may have set up a global filter.
+  set_sources_assignment_filter([])
+
   sources = []
   sources += skia_core_sources
   sources += skia_effects_sources