Correctly mark "Begin SRT download" as deprecated

Bug: 
Change-Id: I934882dd18faee1d999565e5a6661148d52fe20c
Reviewed-on: https://chromium-review.googlesource.com/801291Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Chris Sharp <csharp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520931}

tools/metrics/histograms/enums.xml

@@ -40289,7 +40289,7 @@ would be helpful to identify which type is being sent.
   <int value="4" label="Failed to start"/>
   <int value="5" label="Registry exit code (deprecated)"/>
   <int value="6" label="Reset retries (deprecated)"/>
-  <int value="7" label="Begin SRT download"/>
+  <int value="7" label="Begin SRT download (deprecated)"/>
   <int value="8" label="No browser to show prompt"/>
   <int value="9" label="No local state (deprecated)"/>
   <int value="10" label="No prompt needed"/>