Remove ViewAssistStructure and use the frameworks data structure

As part of the API rename from ViewAssistStructure to ViewStructure,
we added a temporary workaround to prevent build breakage. Remove
the temporary workaround since the current unbundled webview package
implements the updated  onProvideVirtualStructure API.

Change-Id: I13a5b8dee3e856eb585de53a0750bd52c7a909a7