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

client/cmd/isolate/common.go

Index: client/cmd/isolate/common.go
diff --git a/client/cmd/isolate/common.go b/client/cmd/isolate/common.go
index ba0da5b712177523bea62a8b9bfa0ef0a3b8b802..8c96d2916f9fac1933d210d132d5b6d31e753425 100644
--- a/client/cmd/isolate/common.go
+++ b/client/cmd/isolate/common.go
@@ -135,6 +135,12 @@ type loggingFlags struct {
 	EventlogEndpoint string
 }
 
+const eventlogEndpointHelp = `The URL destination for eventlogs. The following special values are supported:
+\t"prod": use the prod eventlog endpoint
+\t"test": use the test eventlog endpoint
+\t"none": disable eventlogging
+\t"auto": use the prod endpoint if --isolate-server is set to the prod isolate server; otherwise disable eventlogging.`
+
 func (lf *loggingFlags) Init(f *flag.FlagSet) {
-	f.StringVar(&lf.EventlogEndpoint, "eventlog-endpoint", "", `The URL destination for eventlogs. The special values "prod" or "test" may be used to target the standard prod or test urls repspectively. An empty string disables eventlogging.`)
+	f.StringVar(&lf.EventlogEndpoint, "eventlog-endpoint", "auto", eventlogEndpointHelp)
 }