Amend dom-distiller script for bug ID handling

"None" would be ignored, and add space after ",".

Bug: None
Change-Id: I85167f2e9ba0cea7affd754825fd34e817568bd5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2304337Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789569}

third_party/dom_distiller_js/update_domdistiller_js.sh

@@ -43,12 +43,12 @@
   # rejoin. Finally, remove the trailing ',' and concat to $bugs.
   git log ${curr_gitsha}..${new_gitsha} \
     | grep -E 'BUG=|Bug:' \
-    | sed -e 's/.*\(BUG=\|Bug:\)\(.*\)/\2/' -e 's/\s*//g' -e '/^$/d' \
+    | sed -e 's/.*\(BUG=\|Bug:\)\(.*\)/\2/' -e 's/\s*//g' -e '/^$/d' -e '/None/d' \
     | tr ',' '\n' \
     | sort \
     | uniq \
-    | tr '\n' ',' \
-    | head --bytes=-1 \
+    | tr '\n' ', ' \
+    | head --bytes=-2 \
     >> $bugs
 
   echo >> $bugs  # add a newline