Cursor doesn't change back to pointer when leaving Mail

Cursor doesn't change back to pointer when leaving Mail
https://bugs.webkit.org/show_bug.cgi?id=132038

Reviewed by Anders Carlsson.

Fixed the bug by adding a cursor rect and making the primary tracking area update the cursor so that
AppKit will reset the cursor as it leaves the WebView.

We use nil cursor here since we manually update the cursor by calling [NSCursor set].

Source/WebKit/mac:

* WebView/WebHTMLView.mm:
(-[WebHTMLView setDataSource:]): Use NSTrackingCursorUpdate option in creating the primary tracking area.

Source/WebKit2:

* UIProcess/API/mac/WKView.mm:
(-[WKView initWithFrame:processPool:configuration:webView:]):
* UIProcess/mac/PageClientImpl.mm:
(WebKit::PageClientImpl::recommendedScrollbarStyleDidChange):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@182026 268f45cc-cd09-0410-ab3c-d52691b4dbfc