Fix a crash which occurs in Windows desktop AURA in the DesktopRootWindowHostWin::IsModalWindowActive function.

Reason dereferencing a NULL root_window_. This function can get called from the WM_NCACTIVATE handler which hits at times
before the root window is created.

Fix is to check for a NULL root window.

BUG=307108
R=sky@chromium.org, sky

Review URL: https://codereview.chromium.org/27407002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228789 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228789