alloy: Fix link click navigation in PDF files (fixes issue #3143)

This change adds a minimal implementation of the |tabs.update| extension API and
modifies StreamsPrivateAPI::SendExecuteMimeTypeHandlerEvent to return a valid
|streamInfo.tabId| value as required by the navigateInCurrentTab implementation
in chrome/browser/resources/pdf/browser_api.js.

libcef/browser/extensions/alloy_extensions_util.h

@@ -0,0 +1,21 @@
+// Copyright 2021 The Chromium Embedded Framework Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be found
+// in the LICENSE file.
+
+#ifndef CEF_LIBCEF_BROWSER_EXTENSIONS_ALLOY_EXTENSIONS_UTIL_H_
+#define CEF_LIBCEF_BROWSER_EXTENSIONS_ALLOY_EXTENSIONS_UTIL_H_
+
+namespace content {
+class WebContents;
+}
+
+namespace extensions {
+namespace alloy {
+
+// Returns the tabId for |web_contents|, or -1 if not found.
+int GetTabIdForWebContents(content::WebContents* web_contents);
+
+}  // namespace alloy
+}  // namespace extensions
+
+#endif  // CEF_LIBCEF_BROWSER_EXTENSIONS_ALLOY_EXTENSIONS_UTIL_H_