[blink] Add Post Increment Operations to WTF::Deque's Iterators

This change adds post increment operations to WTF::Deque's iterators.
Without these operations the iterators fail to fulfill the requirements
for bidirectional iterators [1] they claim to implement, making them
potentially unusable in generic algorithms.

[1] https://en.cppreference.com/w/cpp/named_req/BidirectionalIterator

Bug: None
Change-Id: I72e3371333fe8adcc609c9b05c13693c9e512f6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2584026Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835767}

third_party/blink/renderer/platform/wtf/deque.h

@@ -264,12 +264,23 @@ class DequeIterator : public DequeIteratorBase<T, inlineCapacity, Allocator> {
     Base::Increment();
     return *this;
   }
-  // postfix ++ intentionally omitted
+
+  Iterator operator++(int) {
+    Iterator tmp = *this;
+    ++*this;
+    return tmp;
+  }
+
   Iterator& operator--() {
     Base::Decrement();
     return *this;
   }
-  // postfix -- intentionally omitted
+
+  Iterator operator--(int) {
+    Iterator tmp = *this;
+    --*this;
+    return tmp;
+  }
 };
 
 template <typename T,
@@ -314,12 +325,23 @@ class DequeConstIterator
     Base::Increment();
     return *this;
   }
-  // postfix ++ intentionally omitted
+
+  Iterator operator++(int) {
+    Iterator tmp = *this;
+    ++*this;
+    return tmp;
+  }
+
   Iterator& operator--() {
     Base::Decrement();
     return *this;
   }
-  // postfix -- intentionally omitted
+
+  Iterator operator--(int) {
+    Iterator tmp = *this;
+    --*this;
+    return tmp;
+  }
 };
 
 template <typename T, wtf_size_t inlineCapacity, typename Allocator>

third_party/blink/renderer/platform/wtf/deque_test.cc

@@ -40,6 +40,42 @@ TEST(DequeTest, Basic) {
   EXPECT_EQ(0ul, int_deque.size());
 }
 
+TEST(DequeTest, Iterators) {
+  Deque<int, 2> deque;
+  deque.push_back(0);
+  deque.push_back(1);
+
+  {
+    auto it = deque.begin();
+    EXPECT_EQ(*it, 0);
+    EXPECT_EQ(*++it, 1);
+    EXPECT_EQ(++it, deque.end());
+  }
+
+  {
+    auto it = deque.begin();
+    EXPECT_EQ(*it++, 0);
+    EXPECT_EQ(*it++, 1);
+    EXPECT_EQ(it, deque.end());
+  }
+
+  {
+    const Deque<int, 2>& c_deque = deque;
+    auto c_it = c_deque.begin();
+    EXPECT_EQ(*c_it, 0);
+    EXPECT_EQ(*++c_it, 1);
+    EXPECT_EQ(++c_it, c_deque.end());
+  }
+
+  {
+    const Deque<int, 2>& c_deque = deque;
+    auto c_it = c_deque.begin();
+    EXPECT_EQ(*c_it++, 0);
+    EXPECT_EQ(*c_it++, 1);
+    EXPECT_EQ(c_it, c_deque.end());
+  }
+}
+
 template <wtf_size_t inlineCapacity>
 void CheckNumberSequence(Deque<int, inlineCapacity>& deque,
                          int from,