Remove unnecessary class forward declaration in WebViewImpl.h

As some CLs have passed ownership of member variables from WebViewImpl to own core class,
some member variables have been removed in WebViewImpl.h. Thus, we don't
need to keep forward class declaration in the WebViewImpl.h as well. This
cl removes unnecessary forward declarations.

Additionally there are already unnecessary forward declarations. Those are
removed together.

BUG=N/A

Review URL: https://codereview.chromium.org/225303020

git-svn-id: svn://svn.chromium.org/blink/trunk@171131 bbb929c8-8fbe-4397-9dbb-9b2b20218538