Revert of Enable -Wdeprecated-register. (patchset #5 id:80001 of https://codereview.chromium.org/2780623003/ )

Reason for revert:
Broke os="chromeos" build: crbug.com/705812

Original issue's description:
> Enable -Wdeprecated-register.
>
> No behavior change.
>
> BUG=255186
>
> Review-Url: https://codereview.chromium.org/2780623003
> Cr-Commit-Position: refs/heads/master@{#459973}
> Committed: 76ad12fee7

TBR=dcheng@chromium.org,rouslan@chromium.org,kbr@chromium.org,thakis@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=255186

Review-Url: https://codereview.chromium.org/2778053003
Cr-Original-Commit-Position: refs/heads/master@{#459989}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: f9a94c24489a558b43f6938e86d6c4e69cc52afb

config/compiler/BUILD.gn

@@ -1064,6 +1064,11 @@ 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