Move third_party/base/numerics to core/fxcrt/numerics

Move this directory and shorten the pdfium::base nested namespace to
namespace pdfium.

Bug: pdfium:2127
Change-Id: Ie65bc4c3e69e672ff04e2af0218b6f9fc1584d03
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/116671
Reviewed-by: Thomas Sepez <tsepez@google.com>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/third_party/DEPS b/third_party/DEPS
index 6ffb470..c09184b 100644
--- a/third_party/DEPS
+++ b/third_party/DEPS
@@ -3,5 +3,6 @@
   '+core/fxcrt/fx_coordinates.h',
   '+core/fxcrt/fx_memory.h',
   '+core/fxcrt/fx_system.h',
+  '+core/fxcrt/numerics/safe_math.h',
   '+partition_alloc/pointers',
 ]