Silence MSVC 64-bit build warning c4309 for FrameTree::invalidCount
https://bugs.webkit.org/show_bug.cgi?id=107439

Reviewed by Dimitri Glazkov.

MSVC detects that the constant notFound is truncated on assignment to
invalidCount. Since we never compare them (as it would always fail on
64-bit anyway), just initialize invalidCount directly to -1.

No new tests. No behavior changed.

* page/FrameTree.h:
(FrameTree):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@140433 268f45cc-cd09-0410-ab3c-d52691b4dbfc