Enable -Wdeprecated-register.

No behavior change.

BUG=255186

Review-Url: https://codereview.chromium.org/2780623003
Cr-Original-Commit-Position: refs/heads/master@{#459973}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 76ad12fee73e4512dbb9c7a79ffb6cec33fae6f2

config/compiler/BUILD.gn

@@ -1064,11 +1064,6 @@ config("default_warnings") {
       # also contain a default: branch. Chrome is full of that.
       "-Wno-covered-switch-default",
 
-      # Clang considers the `register` keyword as deprecated, but e.g.
-      # code generated by flex (used in angle) contains that keyword.
-      # http://crbug.com/255186
-      "-Wno-deprecated-register",
-
       # TODO(thakis): This used to be implied by -Wno-unused-function,
       # which we no longer use. Check if it makes sense to remove
       # this as well. http://crbug.com/316352