[Compose] Add buildflag and GN arg for Compose

To prevent sprinkling platform dependencies all ofver the place, we
can use |enable_compose| to trigger our feature's compilation.

Bug: b/303070437
Change-Id: I5b50d555fc6674ca4ecbb2a7fdb648be91651e44
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4902944
Reviewed-by: Peter Williamson <[email protected]>
Commit-Queue: Justin DeWitt <[email protected]>
Auto-Submit: Justin DeWitt <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1204297}
diff --git a/components/compose/BUILD.gn b/components/compose/BUILD.gn
new file mode 100644
index 0000000..831beb3b
--- /dev/null
+++ b/components/compose/BUILD.gn
@@ -0,0 +1,10 @@
+# Copyright 2023 The Chromium Authors
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+import("//build/buildflag_header.gni")
+import("//components/compose/features.gni")
+
+buildflag_header("buildflags") {
+  header = "buildflags.h"
+  flags = [ "ENABLE_COMPOSE=$enable_compose" ]
+}