Migrate to NOTREACHED() in ui/

NOTREACHED() and NOTREACHED_IN_MIGRATION() are both CHECK-fatal now.
The former is [[noreturn]] so this CL also performs dead-code removal
after the NOTREACHED().

This CL does not attempt to do additional rewrites of any surrounding
code, like:

if (!foo) {
  NOTREACHED();
}

to CHECK(foo);

Those transforms take a non-trivial amount of time (and there are
thousands of instances). Cleanup can be left as an exercise for the
reader.

Bug: 40580068
Change-Id: Ib1ec65c775c4c64f0dffc002c232000d3688a2fb
Cq-Include-Trybots: luci.chromium.try:linux-dcheck-off-rel
Low-Coverage-Reason: OTHER Should-be-unreachable code
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5975073
Commit-Queue: Peter Boström <[email protected]>
Reviewed-by: Sam McNally <[email protected]>
Reviewed-by: Mitsuru Oshima <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1375585}
diff --git a/ui/shell_dialogs/select_file_dialog_fuchsia.cc b/ui/shell_dialogs/select_file_dialog_fuchsia.cc
index 62d5953..767f7f6 100644
--- a/ui/shell_dialogs/select_file_dialog_fuchsia.cc
+++ b/ui/shell_dialogs/select_file_dialog_fuchsia.cc
@@ -12,8 +12,7 @@
 SelectFileDialog* CreateSelectFileDialog(
     SelectFileDialog::Listener* listener,
     std::unique_ptr<SelectFilePolicy> policy) {
-  NOTREACHED_IN_MIGRATION();
-  return nullptr;
+  NOTREACHED();
 }
 
 }  // namespace ui