Disentangle pdfium::span<> from unowned_ptr<>.

We need only have the raw_ptr<> or T* variations now that the code
has been consolidated against base::raw_ptr<>. Hence, UnownedPtr can
once again be part of fxcrt, rather than its own component. Then
remove UNOWNED_PTR_IS_BASE_RAW_PTR which is now the same as
PDF_USE_PARTITION_ALLOC.

-- remove pre-existing unused include_dirs while at it.
-- use inline initialization and `= default` while at it.

Change-Id: Ib852cca35cd6de9294efda1c1baaf16e89aa330e
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/112531
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/third_party/DEPS b/third_party/DEPS
index dfd263b..6ffb470 100644
--- a/third_party/DEPS
+++ b/third_party/DEPS
@@ -3,5 +3,5 @@
   '+core/fxcrt/fx_coordinates.h',
   '+core/fxcrt/fx_memory.h',
   '+core/fxcrt/fx_system.h',
-  '+core/fxcrt/unowned_ptr.h',
+  '+partition_alloc/pointers',
 ]