Convert enable_plugins to a buildflag header.

Moves the build flag from build/config/features.gni to ppapi/features/features.gni so it's not global any more.

Moves the define from build/config (global) to a generated buildflag header //ppapi/features:features. Updates callers to use BUILDFLAG, include the new header, and depend on this target.

R=bbudge
TBR=rsesek@chromium.org (*_messages.h files).
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2512563003
Cr-Original-Commit-Position: refs/heads/master@{#433426}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 4b46108b37f0e0c7de0f86f3dc1d229e1a2bf168

config/BUILD.gn

@@ -65,9 +65,6 @@ config("feature_flags") {
   if (enable_notifications) {
     defines += [ "ENABLE_NOTIFICATIONS" ]
   }
-  if (enable_plugins) {
-    defines += [ "ENABLE_PLUGINS=1" ]
-  }
   if (enable_pdf) {
     defines += [ "ENABLE_PDF=1" ]
   }

config/features.gni

@@ -21,7 +21,6 @@ if (is_android) {
 }
 
 declare_args() {
-  enable_plugins = (!is_android && !is_ios) || is_chromecast
   enable_pdf = !is_android && !is_ios && !is_chromecast
 
   # Enables Native Client support.