Extensions: Assert for enable_extensions in c/b/r/extensions/BUILD.gn

This ensures that the BUILD.gn file is not unnecessarily evaluated
where not applicable.

Bug: None
Change-Id: I4bc18a19f6fee782ed85df7a2d007465bedced3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626912
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Auto-Submit: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842967}

chrome/browser/resources/extensions/BUILD.gn

@@ -4,6 +4,7 @@
 
 import("//build/config/chromeos/ui_mode.gni")
 import("//chrome/common/features.gni")
+import("//extensions/buildflags/buildflags.gni")
 import("//third_party/closure_compiler/compile_js.gni")
 import("//tools/grit/grit_rule.gni")
 import("//tools/grit/preprocess_if_expr.gni")
@@ -11,6 +12,8 @@ import("//tools/polymer/html_to_js.gni")
 import("//ui/webui/resources/tools/generate_grd.gni")
 import("../tools/optimize_webui.gni")
 
+assert(enable_extensions, "enable extensions check failed")
+
 preprocess_folder = "preprocessed"
 preprocess_manifest = "preprocessed_manifest.json"
 preprocess_gen_manifest = "preprocessed_gen_manifest.json"