TextFieldInputType::handleBeforeTextInsertedEvent shouldn't use plainText

TextFieldInputType::handleBeforeTextInsertedEvent shouldn't use plainText
https://bugs.webkit.org/show_bug.cgi?id=128953

Reviewed by Alexey Proskuryakov.

Don't use FrameSelection's toNormalizedRange and plainText. Instead, use the cached selection start and selection
end to extract the selected text. The caches are updated inside FrameSelection::setSelection whenever selection
is inside a text form control via HTMLTextFormControlElement::selectionChanged.

* html/TextFieldInputType.cpp:
(WebCore::TextFieldInputType::handleBeforeTextInsertedEvent):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@164329 268f45cc-cd09-0410-ab3c-d52691b4dbfc