Cleanup: Add conversion interface for WebWidget derived classes.

Adding toWebHelperPluginImpl, toWebPagePopupImpl & toWebPopupMenuImpl conversion
functions to replace static_cast, which are derived from WebWidget.

R=tkent

BUG=None

TEST=None

Review URL: https://codereview.chromium.org/26564006

git-svn-id: svn://svn.chromium.org/blink/trunk@159315 bbb929c8-8fbe-4397-9dbb-9b2b20218538