[PartitionAlloc] Split PartitionRoot out of partition_alloc.h

Prepare for https://crrev.com/c/2374465.

Change-Id: I3785f9ac73e7da143e8753dcd86f175cc77c026a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463047
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816438}

base/BUILD.gn

@@ -1779,6 +1779,7 @@ component("base") {
         "allocator/partition_allocator/partition_page.h",
         "allocator/partition_allocator/partition_ref_count.cc",
         "allocator/partition_allocator/partition_ref_count.h",
+        "allocator/partition_allocator/partition_root.h",
         "allocator/partition_allocator/partition_tag.h",
         "allocator/partition_allocator/partition_tag_bitmap.h",
         "allocator/partition_allocator/partition_tls.h",