Cleanup: Remove most enable_extensions logic in chrome/ extensions code.

chrome/common/extensions/api/schemas.gni

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/features.gni")
 
+assert(enable_extensions)
+
 gypi_values = exec_script(
     "//build/gypi_to_gn.py",
     [ rebase_path("schemas.gypi") ],
     "scope",
     [ "schemas.gypi" ])
 
 # Common sources that are both bundled and compiled.
 sources = gypi_values.main_schema_files
 if (is_chromeos) {
   sources += gypi_values.chromeos_schema_files
 }
 if (enable_webrtc) {
   sources += gypi_values.webrtc_schema_files
 }
 
 
 uncompiled_sources = gypi_values.main_non_compiled_schema_files
 
 root_namespace = "extensions::api::%(namespace)s"
 schema_include_rules =
     "extensions/common/api:extensions::core_api::%(namespace)s"
 schema_dependencies = [ "//extensions/common/api" ]