Only reveal selection service UI after a short delay.

Only reveal selection service UI after a short delay.
<rdar://problem/16777346> and https://bugs.webkit.org/show_bug.cgi?id=132418

Reviewed by Sam Weinig.

* WebProcess/WebPage/SelectionOverlayController.cpp:
(WebKit::SelectionOverlayController::SelectionOverlayController):
(WebKit::SelectionOverlayController::destroyOverlay): Also stop the hover timer.
(WebKit::SelectionOverlayController::selectionRectsDidChange): Mark the highlight as dirty.
(WebKit::SelectionOverlayController::hoverTimerFired): If the mouse is still over the highlight,
  set the visible flag and setNeedsDisplay().
* WebProcess/WebPage/SelectionOverlayController.h:

* WebProcess/WebPage/mac/SelectionOverlayControllerMac.mm:
(WebKit::SelectionOverlayController::drawRect): Recreate the highlight if it exists but
  is marked as dirty. After doing that, possibly reset the hover timer.
  Also, only performing the actual drawing if the visible flag is set.
(WebKit::SelectionOverlayController::mouseEvent): If the mouse moves on or off the highlight,
  start or stop the hover timer accordingly.
(WebKit::SelectionOverlayController::mouseHoverStateChanged): Handle starting/stopping the
  hover timer.
(WebKit::SelectionOverlayController::clearHighlight): Deleted.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@168084 268f45cc-cd09-0410-ab3c-d52691b4dbfc