Log uses of isolate archive and exparchive when 'chromium' build tag is set.

client/cmd/isolate/build_chromium.go

Index: client/cmd/isolate/build_chromium.go
diff --git a/client/cmd/isolate/build_chromium.go b/client/cmd/isolate/build_chromium.go
new file mode 100644
index 0000000000000000000000000000000000000000..9a0c426081384897427cc176228728e4da452016
--- /dev/null
+++ b/client/cmd/isolate/build_chromium.go
@@ -0,0 +1,12 @@
+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+// +build chromium
+
+package main
+
+// autoLogProd returns whether to log eventlogs to prod when the --eventlog-endpoint is set to auto.  When we are building for the chromium project, we automatically log usage.
+func autoEventlog() bool {
+	return true
+}