Fieldset NG: Fix hit-testing for the rendered LEGEND, take 2

crrev.com/806924 missed to add |accumulated_offset| to the LEGEND's
offset.  So hit-testing didn't work well if the FIELDSET was placed far
from the origin.

Bug: 1127743
Change-Id: I16c80b8074f9082516a6ecd5bb82407bbe7d3551
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2413681
Auto-Submit: Kent Tamura <tkent@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807797}

html/rendering/non-replaced-elements/the-fieldset-and-legend-elements/legend-hover.html

@@ -9,6 +9,7 @@
   background: lime;
 }
 </style>
+<h1>Header</h1>
 <fieldset>
   <legend>Legend</legend>
 </fieldset>