[protosync] Server-side -> client

Sync server-side changes from
https://critique.corp.google.com/#review/259645392 to client.

Change-Id: Iae12415e75902d0199d84dc0e0b99714609164c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1751248Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Commit-Queue: Jit Yao Yap <jityao@google.com>
Cr-Commit-Position: refs/heads/master@{#686413}

components/policy/proto/device_management_backend.proto

@@ -2258,7 +2258,7 @@ message AppInstallReportLogEvent {
     RESUME = 4;
   }
 
-  // Timestamp, in milliseconds since epoch. Set for all log
+  // Timestamp, in microseconds since epoch. Set for all log
   // events.
   optional int64 timestamp = 1;