Reland "Move GN enable_plugins variable out of //ppapi"

This is a reland of commit 0234539437a5596ef2c5bc9d45629b1d66305ce2

Fix the Chromecast build failure by wrapping enable_plugins in a
declare_args() block, like how it was prior to this CL.

Original change's description:
> Move GN enable_plugins variable out of //ppapi
>
> Move the GN enable_plugins variable to //content/public/common, so it
> lives in the same directory as the C++ ENABLE_PLUGINS buildflag.
>
> Bug: 40511450
> Change-Id: Ic73f19e510b5b5cc94ecda37519de764abd03fba
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6757897
> Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
> Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
> Reviewed-by: Derek Schuff <dschuff@chromium.org>
> Owners-Override: Dave Tapuska <dtapuska@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1487258}

Bug: 40511450
Cq-Include-Trybots: luci.chromium.try:linux-cast-arm-rel
Change-Id: I1ef677b6281278d7af18bcafdea1378098272102
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6758510
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Owners-Override: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1487670}
NOKEYCHECK=True
GitOrigin-RevId: 48ef5791f5d1a0a80734436272d12cb9d10137b1
diff --git a/buildflags/buildflags.gni b/buildflags/buildflags.gni
index 5e23de2..9562eb1 100644
--- a/buildflags/buildflags.gni
+++ b/buildflags/buildflags.gni
@@ -2,18 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//build/config/cast.gni")
-
-declare_args() {
-  # Enables basic plugin support. This enables support for registering and
-  # looking up plugins, but does not include support for loading plugins using a
-  # specific technology like Pepper.
-  #
-  # In particular, the PDF viewer (enable_pdf) requires plugin support, but it
-  # does not require Pepper support (enable_ppapi).
-  enable_plugins = !is_android && !is_ios && !is_castos
-}
-
 declare_args() {
   # Enables Pepper API (PPAPI) plugin support.
   # The only remaining PPAPI plugin supported is NaCl and if is disabled