Return HTCLIENT from the WM_NCHITTEST message handler in the LegacyRenderWidgetHostHWND class
if the parent returns HTNOWHERE

The parent can return HTNOWHERE in its WM_NCHITTEST handler if it is a popup window. For e.g.
a select box, etc. The LegacyRenderWidgetHostHWND should return HTCLIENT in this case to
ensure that we don't get HTVSCROLL/HTHSCROLL being returned via DefWindowProc. This could
happen because the fake scroll styles are present on the window.

We have similar code in the HWNDMessageHandler::OnNCHitTest function which was added as part
of the recent scroll fixes. This needs to be needs to be done in the LegacyRenderWidgetHostHWND class now
as all scroll fixes are being consolidated here.

BUG=347431, 335941
R=cpu@chromium.org, sky@chromium.org, cpu

Review URL: https://codereview.chromium.org/181043011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253930 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253930