service worker: Update description of ContextRequestHandlerStatus UMA

Add notes to describe this is semi-deprecated.

Bug: 852666
Change-Id: If19554c580ba082187364211451a106d275e9d96
Reviewed-on: https://chromium-review.googlesource.com/1128659
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573563}

tools/metrics/histograms/histograms.xml

@@ -89040,6 +89040,12 @@ uploading your change for review.
     The result of ServiceWorkerContextRequestHandler handling a request for a
     service worker script. The histogram is suffixed to distinguish new vs
     installed workers and main vs imported scripts.
+
+    Note that as of M69, this no longer includes most script loads because those
+    are handled by ServiceWorkerInstalledScriptsManager, which doesn't record
+    this histogram.
+
+    It will be removed after NetS13nSW ships. https://crbug.com/715640
   </summary>
 </histogram>