Widget's position change should not initiate layout, only when its size changes.

Widget's position change should not initiate layout, only when its size changes.
https://bugs.webkit.org/show_bug.cgi?id=123860

Reviewed by Andreas Kling.

RenderWidgets initiate unnecessary layouts while scrolling when they are embedded to
overflow:scroll containers. Scroll position change doesn't dirty the render tree
so it should not trigger layout either.

.:

* ManualTests/layouts-on-renderwidgets-while-scrolling.html: Added.

Source/WebCore:

Manual test added. Unfortunately we can't test against the number of layouts yet.

* rendering/RenderWidget.cpp:
(WebCore::RenderWidget::setWidgetGeometry):
(WebCore::RenderWidget::updateWidgetGeometry):
(WebCore::RenderWidget::updateWidgetPosition):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@158727 268f45cc-cd09-0410-ab3c-d52691b4dbfc