Add a chrome://webapks page.

components/webapks_ui/BUILD.gn

Index: components/webapks_ui/BUILD.gn
diff --git a/components/webapks_ui/BUILD.gn b/components/webapks_ui/BUILD.gn
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index 0000000000000000000000000000000000000000..8fb92980430b8bff06fb7be78e7ad305ba310bd5
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+# Copyright 2017 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.
+
+static_library("webapks_ui") {
+  sources = [
+    "webapks_ui_constants.cc",
+    "webapks_ui_constants.h",
+  ]
+
+  deps = [
+    "//base",
+  ]
+}