Refactor fileutils.cc/h and fileutils_unittests.cc into their own targets.

webrtc/build/webrtc.gni

Index: webrtc/build/webrtc.gni
diff --git a/webrtc/build/webrtc.gni b/webrtc/build/webrtc.gni
index 17405c3cb38f8204a15661ca143ff29ef5d412e5..91715225718dc4d40fd6f8f8f1bb859730d64d5a 100644
--- a/webrtc/build/webrtc.gni
+++ b/webrtc/build/webrtc.gni
@@ -14,6 +14,10 @@ import("//build_overrides/build.gni")
 import("//testing/test.gni")
 
 declare_args() {
+  # When set to false, we could replace this with a downstream-specific
+  # implementation
+  rtc_use_default_fileutils = true
+
   # Disable this to avoid building the Opus audio codec.
   rtc_include_opus = true