Fix LayerTreeImpl::IteratorAdapter for clang-10

This makes std::iterator_traits work for IteratorAdapter, which may
be required by some std algorithms.

Change-Id: Ib7bf7d8c7f97e6287006314e3b8958ed53bd6b1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1825467Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699897}

cc/trees/layer_tree_impl.h

@@ -194,7 +194,8 @@ class CC_EXPORT LayerTreeImpl {
   // Adapts an iterator of std::unique_ptr<LayerImpl> to an iterator of
   // LayerImpl*.
   template <typename Iterator>
-  class IteratorAdapter {
+  class IteratorAdapter
+      : public std::iterator<std::forward_iterator_tag, LayerImpl*> {
    public:
     explicit IteratorAdapter(Iterator it) : it_(it) {}
     bool operator==(IteratorAdapter o) const { return it_ == o.it_; }