Remove unnecessary static_cast

Following up a review comment in r837819 crrev.com/c/2589576
that was made after it was landed.

Bug: 1154531
Change-Id: Ib2d6529646be5d4287317fd00bcd193ad10c22c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2595750
Auto-Submit: Koji Ishii <kojii@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837860}

third_party/blink/renderer/core/paint/ng/ng_box_fragment_painter_test.cc

@@ -63,8 +63,7 @@ TEST_P(NGBoxFragmentPainterTest, ScrollHitTestOrder) {
     <div id='scroller'>TEXT</div>
   )HTML");
   auto& scroller = *GetLayoutBoxByElementId("scroller");
-  const DisplayItemClient& root_fragment =
-      static_cast<const DisplayItemClient&>(scroller);
+  const DisplayItemClient& root_fragment = scroller;
 
   NGInlineCursor cursor;
   cursor.MoveTo(*scroller.SlowFirstChild());