Make partition alloc's MetadataAllocator non final.

This class is used as and allocator passed to std::vector. In case of
libstdc++ however the allocator is inherited by std::_Vector_impl which
breaks chromium builds against libstdc++.

Ref:
https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/bits/stl_vector.h#L129

Bug: 957519
Change-Id: I85b6d8160d3be1dc11de28c4d58fbb1510398701
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2485064Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Piotr Tworek <ptworek@vewd.com>
Cr-Commit-Position: refs/heads/master@{#819252}

base/allocator/partition_allocator/pcscan.cc

@@ -41,7 +41,7 @@ ThreadSafePartitionRoot& PCScanMetadataAllocator() {
 // STL allocator which is needed to keep internal data structures required by
 // PCScan.
 template <typename T>
-class MetadataAllocator final {
+class MetadataAllocator {
  public:
   using value_type = T;