partition_alloc: fix missing basic_common_reference in C++17.

A template specialization was introduced by:
https://chromium-review.googlesource.com/c/chromium/src/+/6135992
but it exists only starting from C++20.

In C++17 with Skia embedder, we get the error:
third_party/externals/partition_alloc/src/partition_alloc/pointers/raw_ptr.h:1274:8: error: ‘basic_common_reference’ is not a class template

Bug: 351867706
Change-Id: Id246985a497c5435663642006a5dcbd1466399e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6276490
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Auto-Submit: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Benoit Lize <lizeb@chromium.org>
Commit-Queue: Benoit Lize <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1421052}

base/allocator/partition_allocator/src/partition_alloc/pointers/raw_ptr.h

@@ -1255,6 +1255,7 @@ struct pointer_traits<::raw_ptr<T, Traits>> {
   }
 };
 
+#if PA_BUILDFLAG(ASSERT_CPP_20)
 // Mark `raw_ptr<T>` and `T*` as having a common reference type (the type to
 // which both can be converted or bound) of `T*`. This makes them satisfy
 // `std::equality_comparable`, which allows usage like:
@@ -1267,23 +1268,20 @@ struct pointer_traits<::raw_ptr<T, Traits>> {
 // error about being unable to invoke `std::ranges::equal_to()`.
 template <typename T,
           base::RawPtrTraits Traits,
-          template <typename>
-          typename TQ,
-          template <typename>
-          typename UQ>
+          template <typename> typename TQ,
+          template <typename> typename UQ>
 struct basic_common_reference<raw_ptr<T, Traits>, T*, TQ, UQ> {
   using type = T*;
 };
 
 template <typename T,
           base::RawPtrTraits Traits,
-          template <typename>
-          typename TQ,
-          template <typename>
-          typename UQ>
+          template <typename> typename TQ,
+          template <typename> typename UQ>
 struct basic_common_reference<T*, raw_ptr<T, Traits>, TQ, UQ> {
   using type = T*;
 };
+#endif  // PA_BUILDFLAG(ASSERT_CPP_20)
 
 }  // namespace std