Make ScrollView::paint() clip by visibleContentRect

      https://bugs.webkit.org/show_bug.cgi?id=108888

Reviewed by Levi Weintraub.

When applyPageScaleFactorInCompositor or fixedVisibleContentRect
are used, frameRect() and visibleContentRect(true).size() are
no longer synonyms, and the latter is the one that should be
used for clipping paints.

New WebFrameTest: pageScaleFactorScalesPaintClip.

Source/WebCore:

* platform/ScrollView.cpp:
(WebCore::ScrollView::paint):

Source/WebKit/chromium:

* tests/WebFrameTest.cpp:


git-svn-id: svn://svn.chromium.org/blink/trunk@142045 bbb929c8-8fbe-4397-9dbb-9b2b20218538