Fix QuicIntervalDeque::Iterator::operator+=

The code was referring to a member which didn't exist. This was uncovered by a
recent Clang change [1] which made the compiler lookup these names at template
parse time rather than instantiation time.

 1. llvm/llvm-project#90152

PiperOrigin-RevId: 630952202

Author: QUICHE team

quiche/quic/core/quic_interval_deque.h

@@ -198,12 +198,12 @@ class QUICHE_NO_EXPORT QuicIntervalDeque {
     Iterator operator+(difference_type amount) const {
       Iterator copy = *this;
       copy.index_ += amount;
-      QUICHE_DCHECK(copy.index_ < copy.deque_->size());
+      QUICHE_DCHECK(copy.index_ < copy.deque_->Size());
       return copy;
     }
     Iterator& operator+=(difference_type amount) {
       index_ += amount;
-      QUICHE_DCHECK(index_ < deque_->size());
+      QUICHE_DCHECK(index_ < deque_->Size());
       return *this;
     }
     difference_type operator-(const Iterator& rhs) const {