[Qt] Fix a crash on startup in FrameLoaderClientQt::callPolicyFunction with MSVC

Reviewed by Allan Sandfeld Jensen.

It's not clear why it crashes, but here's my guess by looking at the generated
assembly code:
Since we're using a member function pointer to PolicyChecker, the representation
is not necessarily just a simple pointer and might be a bigger structure used
to handle virtual functions resolution.
For a function taking such a function "pointer" as parameter, the compiler doesn't
seem to have the caller and callee agree on which convention to use if the function
pointer's class is non-virtual and only one of the ends doesn't know about it.

Including the definition of the member function pointer's class fixes the crash.

* WebCoreSupport/FrameLoaderClientQt.cpp:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150021 268f45cc-cd09-0410-ab3c-d52691b4dbfc