extensions: Move RecordRequestHeader and RecordResponseHeader to fixed_flat_map.

Rather than copy to the heap to make a dynamically modifiable flat_map,
we can build the map into .data.rel.ro at compile time.

To support current static_assert, base::flat_tree::size is made
constexpr if the underlying collection is, as it is for fixed_flat_map
and fixed_flat_set (specifically, std::array).
For consistency, max_size and empty are similarly updated.

Bug: None
Change-Id: Ia21850b48e928e328f9cb85702b561925cca2d3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2566034Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Auto-Submit: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832358}

base/containers/flat_tree.h

@@ -254,9 +254,9 @@ class flat_tree {
 
   void clear();
 
-  size_type size() const;
-  size_type max_size() const;
-  bool empty() const;
+  constexpr size_type size() const;
+  constexpr size_type max_size() const;
+  constexpr bool empty() const;
 
   // --------------------------------------------------------------------------
   // Iterators.
@@ -681,19 +681,21 @@ void flat_tree<Key, GetKeyFromValue, KeyCompare, Container>::clear() {
 }
 
 template <class Key, class GetKeyFromValue, class KeyCompare, class Container>
-auto flat_tree<Key, GetKeyFromValue, KeyCompare, Container>::size() const
-    -> size_type {
+constexpr auto flat_tree<Key, GetKeyFromValue, KeyCompare, Container>::size()
+    const -> size_type {
   return body_.size();
 }
 
 template <class Key, class GetKeyFromValue, class KeyCompare, class Container>
-auto flat_tree<Key, GetKeyFromValue, KeyCompare, Container>::max_size() const
+constexpr auto
+flat_tree<Key, GetKeyFromValue, KeyCompare, Container>::max_size() const
     -> size_type {
   return body_.max_size();
 }
 
 template <class Key, class GetKeyFromValue, class KeyCompare, class Container>
-bool flat_tree<Key, GetKeyFromValue, KeyCompare, Container>::empty() const {
+constexpr bool flat_tree<Key, GetKeyFromValue, KeyCompare, Container>::empty()
+    const {
   return body_.empty();
 }