Fix background tracing enum descriptions

Change-Id: I305074648b3265c34ec5a52e625e772991363398
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2293228
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Commit-Queue: oysteine <oysteine@chromium.org>
Auto-Submit: ssid <ssid@chromium.org>
Reviewed-by: oysteine <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#787447}

tools/metrics/histograms/enums.xml

diff --git a/tools/metrics/histograms/enums.xml b/tools/metrics/histograms/enums.xml
index f9e56d7..2a51448 100644
--- a/tools/metrics/histograms/enums.xml
+++ b/tools/metrics/histograms/enums.xml
@@ -5937,7 +5937,8 @@
   <int value="11" label="Trace upload succeeded"/>
   <int value="12" label="Startup scenario triggered"/>
   <int value="13" label="Large upload waiting to retry"/>
-  <int value="14" label="Reached code scenario triggered"/>
+  <int value="14" label="System scenario triggered"/>
+  <int value="15" label="Reached code scenario triggered"/>
 </enum>
 
 <enum name="BackingStoreResults">