Update enums.xml with missing ChromeDownloadSource entries.

This CL just updates enums.xml's definition of ChromeDownloadSource to
match the one in the code.

Change-Id: I0160d8986f6c542f58bac2891b55089d7f6e6a99
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1891016
Auto-Submit: Joe DeBlasio <jdeblasio@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710932}

tools/metrics/histograms/enums.xml

@@ -7918,6 +7918,8 @@ histogram as enum -->
   <int value="2" label="Initiated by WebStore Installer"/>
   <int value="3" label="Initiated by ImageBurner"/>
   <int value="4" label="Initiated by Plugin Installer"/>
+  <int value="5" label="Initiated by PDF plugin"/>
+  <int value="6" label="Initiated by Extension"/>
 </enum>
 
 <enum name="ChromeHomeOpenReason">