[gn] Allow overriding sanitizer suppression files

Port https://codereview.chromium.org/2267753002/

BUG=474921
TBR=jochen@chromium.org, kjellander@chromium.org, vogelheim@chromium.org

Review-Url: https://codereview.chromium.org/2290213002
Cr-Commit-Position: refs/heads/master@{#39009}

build_overrides/build.gni

@@ -16,3 +16,8 @@ build_with_chromium = false
 
 # Some non-Chromium builds don't support building java targets.
 enable_java_templates = false
+
+# Allows different projects to specify their own suppressions files.
+asan_suppressions_file = "//build/sanitizers/asan_suppressions.cc"
+lsan_suppressions_file = "//build/sanitizers/lsan_suppressions.cc"
+tsan_suppressions_file = "//build/sanitizers/tsan_suppressions.cc"