Use jinja2 to template webview manifest files.

android_webview/apk/system_webview_apk_common.gypi

Index: android_webview/apk/system_webview_apk_common.gypi
diff --git a/android_webview/apk/system_webview_apk_common.gypi b/android_webview/apk/system_webview_apk_common.gypi
index 94c730c168023fc45b2387f3279591ebfb28b4cf..359d94741295bca91419c11bebf2c69572e95737 100644
--- a/android_webview/apk/system_webview_apk_common.gypi
+++ b/android_webview/apk/system_webview_apk_common.gypi
@@ -24,6 +24,10 @@
     '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',
@@ -32,6 +36,7 @@
     ],
     'includes': [
       '../snapshot_copying.gypi',
+      '../../build/android/jinja_template.gypi',
     ],
     'conditions': [
       ['icu_use_data_file_flag==1', {