Add server generated token to DeepScanningClientResponse

To distinguish multiple, parallel requests from the same user, we need
a token that is per-request. This is added to the
DeepScanningClientResponse, and returned when the user first uploads a
file. It will then be passed within each FCM message to identify which
request the message corresponds to.

Bug: 980784
Change-Id: Ic8176ea21b891187a6a346d0c58dfe9d466bb7bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715540
Auto-Submit: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Commit-Queue: Varun Khaneja <vakh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680207}

components/safe_browsing/proto/webprotect.proto

@@ -61,4 +61,8 @@ message DeepScanningClientResponse {
 
   // DLP scan specific response info.
   optional DlpDeepScanningClientResponse dlp_scan_response = 2;
+
+  // Token used to correlate requests and responses. This is different than the
+  // FCM token, in that it is unique for each request.
+  optional string token = 3;
 }