Eliminate internals.setMockScrollbarsEnabled()

Eliminate internals.setMockScrollbarsEnabled()
https://bugs.webkit.org/show_bug.cgi?id=132085

Reviewed by Tim Horton.

Source/WebCore:
This was essentially unused, and also didnt work.

* testing/InternalSettings.cpp:
(WebCore::InternalSettings::Backup::restoreTo):
(WebCore::InternalSettings::setMockScrollbarsEnabled): Deleted.
* testing/InternalSettings.h:
* testing/InternalSettings.idl:

Tools:
* DumpRenderTree/mac/DumpRenderTree.mm: (resetWebPreferencesToConsistentValues):
No need to reset the preference, there is no way it can change.

LayoutTests:
Remove calls, all of which were resetting it to true (which is the default, and
the only value that makes sense).

* compositing/geometry/fixed-position.html:
* compositing/geometry/horizontal-scroll-composited.html:
* compositing/geometry/vertical-scroll-composited.html:
* compositing/iframes/resources/scrollgrandchild-inner.html:
* compositing/overflow/fixed-position-ancestor-clip.html:
* compositing/resources/mock_scrollbars.js: Removed.
* css3/filters/blur-filter-page-scroll-parents.html:
* css3/filters/blur-filter-page-scroll-self.html:
* css3/filters/blur-filter-page-scroll.html:
* fast/dom/window-scroll-scaling.html:
* fast/events/touch/gesture/gesture-scrollbar.html:
* fast/sub-pixel/sub-pixel-iframe-copy-on-scroll.html:
* fast/sub-pixel/transformed-iframe-copy-on-scroll.html:
* media/video-controls-rendering.html:
* media/video-zoom.html:
* scrollingcoordinator/resources/non-fast-scrollable-region-testing.js:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@167732 268f45cc-cd09-0410-ab3c-d52691b4dbfc