Split out base/notimplemented.h

This is not an invariant, and should probably not be bundled with
NOTREACHED().

Fix IWYU in base/, leave TODO for the rest.

Bug: 1520664
Change-Id: I4e1e811d826f073fc9e559399ab90fb21afb7582
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5204888
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1250583}

diff --git a/base/OWNERS b/base/OWNERS
index 9501e57..255a11e 100644
--- a/base/OWNERS
+++ b/base/OWNERS

@@ -41,6 +41,8 @@
 per-file dcheck*=pbos@chromium.org
 per-file logging*=olivierli@chromium.org
 per-file logging*=pbos@chromium.org
+per-file notimplemented.h=olivierli@chromium.org
+per-file notimplemented.h=pbos@chromium.org
 per-file notreached.h=olivierli@chromium.org
 per-file notreached.h=pbos@chromium.org