Use jinja2 to template webview manifest files.

android_webview/apk/system_webview_apk_common.gypi

 # 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.
 #
 # When making changes remember that this is shared with the internal .apk
 # build rules.
 {
   'type': 'none',
   'dependencies': [
     '<(DEPTH)/android_webview/android_webview.gyp:libwebviewchromium',
     '<(DEPTH)/android_webview/android_webview.gyp:android_webview_java',
     '<(DEPTH)/android_webview/android_webview.gyp:android_webview_pak',
   ],
   'variables': {
     'native_lib_target': 'libwebviewchromium',
     'never_lint': 1,
     'R_package': 'com.android.webview.chromium',
     'R_package_relpath': 'com/android/webview/chromium',
     'shared_resources': 1,
     'extensions_to_not_compress': 'pak,bin,dat',
     'asset_location': '<(INTERMEDIATE_DIR)/assets/',
     'snapshot_copy_files': '<(snapshot_copy_files)',
     # TODO: crbug.com/442348 Update proguard.flags and re-enable.
     'proguard_enabled': 'false',
     'proguard_flags_paths': ['<(DEPTH)/android_webview/apk/java/proguard.flags'],
     # TODO: crbug.com/405035 Find a better solution for WebView .pak files.

[Torne, 2015/03/30 14:02:38]

This TODO applies to the additional_input_paths section below, you should insert
your extra lines somewhere else :)

[Tobias Sargeant, 2015/03/30 15:54:14]

Done.

+    'jinja_inputs': ['<(android_manifest_template_path)'],
+    'jinja_output': '<(INTERMEDIATE_DIR)/AndroidManifest.xml',
+    'jinja_variables': '<(android_manifest_template_vars)',
+    'android_manifest_path': '<(jinja_output)',
     'additional_input_paths': [
       '<@(webview_locales_output_paks)',
       '<(asset_location)/webviewchromium.pak',
       '<(asset_location)/webview_licenses.notice',
       '<@(snapshot_additional_input_paths)',
     ],
     'includes': [
       '../snapshot_copying.gypi',
+      '../../build/android/jinja_template.gypi',
     ],
     'conditions': [
       ['icu_use_data_file_flag==1', {
         'additional_input_paths': [
           '<(asset_location)/icudtl.dat',
         ],
       }],
     ],
   },
   'copies': [
@@ skipping 31 matching lines @@
         '<(asset_location)/webview_licenses.notice',
       ],
       'message': 'Generating WebView license notice',
     },
   ],
   'includes': [
     'system_webview_locales_paks.gypi',
     '../../build/java_apk.gypi',
   ],
 }