Revert of Enable -Wdeprecated-register (except on CrOS). (patchset #7 id:120001 of https://codereview.chromium.org/2780623003/ )

Reason for revert:

Findit identified CL at revision 460879 as the culprit for
failures in the build cycles as shown on:
https://findit-for-me.appspot.com/waterfall/culprit?key=ag9zfmZpbmRpdC1mb3ItbWVyRAsSDVdmU3VzcGVjdGVkQ0wiMWNocm9taXVtL2E0Njg3MmNhOTZkODYyODE2Zjk5M2MxNmQ2YzI3MGFmMjYyOTAyNDcM

Original issue's description:
> Enable -Wdeprecated-register (except on CrOS).
>
> No behavior change.
>
> BUG=255186,705812
>
> Review-Url: https://codereview.chromium.org/2780623003
> Cr-Original-Commit-Position: refs/heads/master@{#459973}
> Committed: 76ad12fee7
> Review-Url: https://codereview.chromium.org/2780623003
> Cr-Commit-Position: refs/heads/master@{#460879}
> Committed: a46872ca96

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=255186,705812

Review-Url: https://codereview.chromium.org/2786273002
Cr-Original-Commit-Position: refs/heads/master@{#460910}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 0c64e3912571118d628f513ccc3a7eb6224099d8

config/compiler/BUILD.gn

@@ -1059,6 +1059,11 @@ 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
@@ -1068,14 +1073,6 @@ config("default_warnings") {
       "-Wno-inconsistent-missing-override",
     ]
 
-    if (is_chromeos) {
-      cflags += [
-        # TODO(thakis): Figure out why CrOS needs this, fix, remove.
-        # https://crbug.com/806812
-        "-Wno-deprecated-register",
-      ]
-    }
-
     # Chrome's hermetic Clang compiler, NaCl's Clang compiler and Xcode's Clang
     # compiler will almost always have different versions. Certain flags may not
     # be recognized by one version or the other.