[PartitionAlloc] Fix and unify alignment requirements for the root.

The alignment directives were not unified across platforms, and didn't
compile on Linux (due to a missing semicolon). Use "alignas()"
everywhere, which is also semantically closer to what we want.

Bug: 1141752
Change-Id: I3553932ac3a0fdeee989eba4b8eae83ffa281098
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2498466Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820809}

base/allocator/allocator_shim_default_dispatch_to_partition_alloc.cc

@@ -36,13 +36,8 @@ namespace {
 std::atomic<bool> g_initialization_lock;
 std::atomic<base::ThreadSafePartitionRoot*> g_root_;
 // Buffer for placement new.
-#if defined(OS_WIN)
-__declspec(align(base::kAlignment)) uint8_t
+alignas(base::ThreadSafePartitionRoot) uint8_t
     g_allocator_buffer[sizeof(base::ThreadSafePartitionRoot)];
-#else
-uint8_t g_allocator_buffer[sizeof(base::ThreadSafePartitionRoot)]
-    __attribute__((aligned(base::kAlignment)))
-#endif
 
 base::ThreadSafePartitionRoot* Allocator() {
   // Double-checked locking.