Adding the ability to move selection by granularity on iOS.

Adding the ability to move selection by granularity on iOS.
https://bugs.webkit.org/show_bug.cgi?id=144410
rdar://problem/19213337

Reviewed by Benjamin Poulain.

This patch also correctly sets _usingGestureForSelection for all
the functions that change the selection with a gesture.
This way we make sure that the selection drawing is not delayed
until the next time we paint.

* UIProcess/WebPageProxy.h:
* UIProcess/ios/WKContentViewInteraction.mm:
(-[WKContentView moveSelectionAtBoundary:inDirection:completionHandler:]):
* UIProcess/ios/WebPageProxyIOS.mm:
(WebKit::WebPageProxy::moveSelectionAtBoundaryWithDirection):
* WebProcess/WebPage/WebPage.h:
* WebProcess/WebPage/WebPage.messages.in:
* WebProcess/WebPage/ios/WebPageIOS.mm:
(WebKit::WebPage::moveSelectionAtBoundaryWithDirection):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@183592 268f45cc-cd09-0410-ab3c-d52691b4dbfc