[Runtime] Move ownership of XWalkContentBrowserClient to XWalkRunner

This is mostly so we take control of BrowserMainParts, that has both
important hook points and currently own different subsystems.

Our browser test infrastructure was also changed to create a XWalkRunner
object instead of a XWalkContentBrowserClient. This is necessary because
some pieces of the client (including main parts) will depend on
XWalkRunner in the future.

XWalkBrowserMainPartsAndroid was duplicating the logic for creating
ContentBrowserClient, this commit removes the override because it
can use the base class implementation.

Embedded in this commit is a small cleanup: removed unnecessary
forwarding declarations and reset() calls to scoped pointers in
XWalkMainDelegate.