Reland of move enable_vulkan from build/config, and move it to the buildflag_header system. (patchset #1 id:1 of https://codereview.chromium.org/2742263004/ )

Reason for revert:
Relanding since this revert doesn't appear to have helped:
https://build.chromium.org/p/chromium.linux/builders/Linux%20Tests

Original issue's description:
> Revert of Remove enable_vulkan from build/config, and move it to the buildflag_header system. (patchset #8 id:140001 of https://codereview.chromium.org/2744973002/ )
>
> Reason for revert:
> Speculative revert for linux tests failure:
> https://build.chromium.org/p/chromium.linux/builders/Linux%20Tests
>
> Original issue's description:
> > Remove enable_vulkan from build/config, and move it to the buildflag_header system.
> >
> > No intended behavior change.
> >
> > TBR=avi
> > BUG=none
> > CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
> >
> > Review-Url: https://codereview.chromium.org/2744973002
> > Cr-Commit-Position: refs/heads/master@{#456525}
> > Committed: 7f118188c1
>
> TBR=dyen@chromium.org,danakj@chromium.org,jbauman@chromium.org,avi@chromium.org,piman@chromium.org,thakis@chromium.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=none
>
> Review-Url: https://codereview.chromium.org/2742263004
> Cr-Commit-Position: refs/heads/master@{#456562}
> Committed: 0192171168

TBR=dyen@chromium.org,danakj@chromium.org,jbauman@chromium.org,avi@chromium.org,piman@chromium.org,thakis@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=none

Review-Url: https://codereview.chromium.org/2746363002
Cr-Original-Commit-Position: refs/heads/master@{#456613}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: db0a348759ea9c97783c516344a1d96880ceb359

config/ui.gni

@@ -35,9 +35,6 @@ declare_args() {
 
   # Whether we should use glib, a low level C utility library.
   use_glib = is_linux
-
-  # Enable experimental vulkan backend.
-  enable_vulkan = false
 }
 
 declare_args() {