DeviceManagementBackend: Add extension request timestamp.

Add field to log when the extension request is made.

Bug: 1006899
Change-Id: I21093c7ede9a6d1e823abf302be50c04973a43f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1930182
Auto-Submit: Alex Vallee <avallee@chromium.org>
Reviewed-by: Owen Min <zmin@chromium.org>
Commit-Queue: Owen Min <zmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718306}

components/policy/proto/device_management_backend.proto

@@ -1174,6 +1174,9 @@ message ChromeSignedInUser {
 message ExtensionRequest {
   // ID of the installed app/extension for a Chrome app or extension.
   optional string id = 1;
+
+  // When the user commits to requesting the extension.
+  optional int64 request_timestamp = 2;
 }
 
 // Extension information.