Upstream: Add WebAPK's client library.

chrome/android/webapk/libs/runtime_library/src/README

Index: chrome/android/webapk/libs/runtime_library/src/README
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+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.
+
+As the runtime library code does not execute in any of Chrome's processes, and
+runs in a separate application/process, it must not depend on anything in Chrome
+including //base.
+
+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.