Refactor WebHelperPluginImpl to simplify plugin creation.

As it turns out, it's not necessary to create a Page, or even a WebFrame
at all. This was problematic for the current work to move embedder
functionality from WebViewClient to WebFrameClient since it entangled
two WebFrames with the same WebFrameClient.

A bonus side effect is the code gets a lot simpler: WebHelperPlugin will
no longer needs to be a WebWidget, and it doesn't need to carefully
construct and destruct its internal state. This allows logic specific to
WebHelperPlugin to be completely removed from WebView.

After this change rolls into Chrome, WebHelperPlugin will be completely
removed from the embedding API, as Chrome no longer needs to interact
with it at all.

BUG=346543
R=abarth@chromium.org, ddorwin@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=167830

Review URL: https://codereview.chromium.org/175593008

git-svn-id: svn://svn.chromium.org/blink/trunk@167849 bbb929c8-8fbe-4397-9dbb-9b2b20218538