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
[email protected] (*_messages.h files).
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2512563003
Cr-Commit-Position: refs/heads/master@{#433426}
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index 000563b..bbc9f2d 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -1450,6 +1450,7 @@
     "//mojo/public/js",
     "//net:extras",
     "//net:net_with_v8",
+    "//ppapi/features",
     "//printing/features",
     "//services/image_decoder/public/cpp",
     "//services/service_manager/public/cpp",