Add mathml/ to the blink_html watchlist category.

Change-Id: Iecc102fca57b21132085671f43e597ae43263bab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1904618
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713866}

WATCHLISTS

@@ -390,7 +390,8 @@
                   '|base/allocator/partition_allocator/'
     },
     'blink_html': {
-      'filepath': 'third_party/blink/renderer/core/html/'
+      'filepath': 'third_party/blink/renderer/core/html/' \
+                  '|third_party/blink/renderer/core/mathml/'
     },
     'blink_htmlparser': {
       'filepath': 'third_party/blink/renderer/core/html/parser/'