[Code Health] Add base::Value::List::rbegin()/rend()

This change adds reverse iterators to base::Value::List to enable more
migrations from std::vector<base::Value> to base::Value::List.

Also updates one call site from using a span now that reverse iterators
are available.

Bug: 1187001
Change-Id: I220857d4bb5dd531ef818ecc5e895ea31a5965ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4539691
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Reviewed-by: Wes Okuhara <wesokuhara@google.com>
Cr-Commit-Position: refs/heads/main@{#1147865}

diff --git a/base/values.h b/base/values.h
index 930ff25..5d62e8c 100644
--- a/base/values.h
+++ b/base/values.h

@@ -140,6 +140,8 @@
 //       `front()`, `back()`, `reserve()`, `operator[]`, `clear()`, `erase()`:
 //       Identical to the STL container equivalents, with additional safety
 //       checks, e.g. `operator[]` will `CHECK()` if the index is out of range.
+// - `rbegin()` and `rend()` are also supported, but there are no safety checks
+// (see crbug.com/1446739).
 // - `Clone()`: Create a deep copy.
 // - `Append()`: Append a value to the end of the list. Accepts `Value` or any
 //       of the subtypes that `Value` can hold.
@@ -628,6 +630,15 @@
     const_iterator end() const;
     const_iterator cend() const;
 
+    // Returns a reverse iterator preceding the first value in this list. May
+    // not be dereferenced.
+    std::vector<Value>::reverse_iterator rend();
+    std::vector<Value>::const_reverse_iterator rend() const;
+
+    // Returns a reverse iterator to the last value in this list.
+    std::vector<Value>::reverse_iterator rbegin();
+    std::vector<Value>::const_reverse_iterator rbegin() const;
+
     // Returns a reference to the first value in the container. Fails with
     // `CHECK()` if the list is empty.
     const Value& front() const;