Don't ever call ::SetCursor(0)

Don't ever call ::SetCursor(0)

Doing so makes the cursor disappear from the screen.

Fixes <http://webkit.org/b/52024> Mouse cursor flashes when moving it
around a WKView before the web process has finished launching

Reviewed by Jon Honeycutt.

* UIProcess/win/WebView.cpp:
(WebKit::WebView::onSetCursor): If we don't have a cursor to set, just
let Windows do its default thing (which is to set the cursor to the
window class's cursor, which in our case is the arrow cursor).
(WebKit::WebView::cursorToShow): Just return 0 when the page has
crashed so that we'll show the default cursor (which is the arrow
cursor). This change has no visible effect, but seems slightly better
conceptually.
(WebKit::WebView::updateNativeCursor): Don't do anything if we don't
have a cursor to set.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@75268 268f45cc-cd09-0410-ab3c-d52691b4dbfc