Add a temporary wrapper for ViewStructure until Webview APK is updated

It turned out that after the API change from ViewAssistStructure to
ViewStructure, the suggested mechanism did not work, and webview
started throwing abstractmethoderror exceptions. Temporarily
solve the problem by wrapping ViewStructure inside a
ViewAssistStructure. Once Webview APK is updated, drop it.

Change-Id: I09dfe7dac9c2bc7c037d842844c61dd879629470