Clean up an Oilpan-related TODO(825895)

As discussed offline, this usage is Oilpan-safe. As I had a few other
attempts without making it simpler (e.g., without using Member<>), I'll
simply remove this TODO.

Bug: 825895
Change-Id: Id677e59625d09a9888e98fac853da734d9d83669
NoTry: True
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1832528Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702065}

third_party/blink/renderer/core/css/css_math_expression_node.h

@@ -324,7 +324,6 @@ class CSSMathExpressionVariadicOperation final : public CSSMathExpressionNode {
 
  private:
   // Helper for iterating from the 2nd to the last operands
-  // TODO(crbug.com/825895): Is this Oilpan-safe?
   base::span<const Member<const CSSMathExpressionNode>> SecondToLastOperands()
       const {
     return base::make_span(std::next(operands_.begin()), operands_.end());