Generate license file separately

So,
The license file doesn't need to be generated for each apk.
The file could reused by downstream apk.

BUG=546219

Review URL: https://codereview.chromium.org/1412633004

Cr-Commit-Position: refs/heads/master@{#357185}

android_webview/android_webview.gyp

@@ -126,6 +126,24 @@
             '<@(_outputs)',
           ],
         },
+        {
+          'action_name': 'generate_webview_license_notice',
+          'inputs': [
+            '<!@(python <(DEPTH)/android_webview/tools/webview_licenses.py notice_deps)',
+            '<(DEPTH)/android_webview/tools/licenses_notice.tmpl',
+            '<(DEPTH)/android_webview/tools/webview_licenses.py',
+          ],
+          'outputs': [
+             '<(webview_licenses_path)',
+          ],
+          'action': [
+            'python',
+              '<(DEPTH)/android_webview/tools/webview_licenses.py',
+              'notice',
+              '<(webview_licenses_path)',
+          ],
+          'message': 'Generating WebView license notice',
+        },
       ],
     },
     # GN version:  //android_webview/locale_paks 

android_webview/apk/system_webview_apk_common.gypi

@@ -46,7 +46,8 @@
     {
       'destination': '<(asset_location)',
       'files': [
-        '<(PRODUCT_DIR)/android_webview_assets/webviewchromium.pak',
+        '<(webview_licenses_path)',
+        '<(webview_chromium_pak_path)',
         '<@(snapshot_copy_files)',
       ],
       'conditions': [
@@ -58,27 +59,8 @@
       ],
     },
   ],
-  'actions': [
-    {
-      'action_name': 'generate_webview_license_notice',
-      'inputs': [
-        '<!@(python <(DEPTH)/android_webview/tools/webview_licenses.py notice_deps)',
-        '<(DEPTH)/android_webview/tools/licenses_notice.tmpl',
-        '<(DEPTH)/android_webview/tools/webview_licenses.py',
-      ],
-      'outputs': [
-        '<(asset_location)/webview_licenses.notice',
-      ],
-      'action': [
-        'python',
-        '<(DEPTH)/android_webview/tools/webview_licenses.py',
-        'notice',
-        '<(asset_location)/webview_licenses.notice',
-      ],
-      'message': 'Generating WebView license notice',
-    },
-  ],
   'includes': [
+    'system_webview_paks.gypi',
     '../../build/java_apk.gypi',
     '../../build/android/jinja_template.gypi',
   ],

android_webview/apk/system_webview_paks.gypi

@@ -9,6 +9,7 @@
 # once Chrome includes all the locales that the WebView needs.
 {
   'variables': {
+    'webview_licenses_path': '<(PRODUCT_DIR)/android_webview_assets/webview_licenses.notice',
     'webview_chromium_pak_path': '<(PRODUCT_DIR)/android_webview_assets/webviewchromium.pak',
     'webview_locales_input_paks_folder': '<(PRODUCT_DIR)/android_webview_assets/locales/',
     # The list of locale are only supported by WebView.