The runtime library contains the implementation of the Web APK service. It
implements functionality which Chrome delegates to the Web APK such as
displaying notifications. The library is loaded by a ShellAPK which
contains the Web APK's manifest and code to load the runtime library.

The latest version of the runtime library is
extracted from Chrome's APK by the ShellAPK when the Web APK service is created.
Since the ShellAPK is updated asynchronously, the runtime library may run within
an old ShellAPK. For instance, the ShellAPK's Android manifest may be missing
some of the permissions that the runtime library is expecting.

The number in runtime_library_version.gni must be incremented whenever the
runtime library is updated. The WebAPK re-extracts the runtime library from the
Chrome APK when the version number is incremented.
