Add description for ServiceWorker.ScriptSize histogram

Blink-side change: https://codereview.chromium.org/524553002/

BUG=408451

Review URL: https://codereview.chromium.org/525733004

Cr-Commit-Position: refs/heads/master@{#293058}

tools/metrics/histograms/histograms.xml

@@ -28734,6 +28734,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="ServiceWorker.ScriptSize" units="bytes">
+  <owner>ksakamoto@chromium.org</owner>
+  <summary>
+    The length of Service Worker scripts. Logged on each load of Service Worker
+    script. It doesn't include the size of imported scripts.
+  </summary>
+</histogram>
+
 <histogram name="SessionCrashed.Bubble" enum="SessionCrashedBubbleUserAction">
   <owner>yiyaoliu@chromium.org</owner>
   <summary>How did the user interact with the SessionCrashed Bubble?</summary>