[WK merge] SVG gradients, patterns and filters do not respect the device scale factor

This is a merge of http://trac.webkit.org/changeset/154856 by
Timothy Horton <timothy_horton@apple.com>.

Blink no longer suffers from this in the case of masking and clipping,
but the problem still exists for filters/patterns/gradients.

Ref-test results do not match perfectly (numerical stability?), using a
pixel test instead.

--------

    calculateTransformationToOutermostCoordinateSystem should include
    the page's device scale factor in its transform.

    rendering/svg/SVGRenderingContext.cpp:
    (WebCore::SVGRenderingContext::calculateTransformationToOutermostCoordinateSystem):

    Add a test that ensures that when deviceScaleFactor=2, masks,
    clipping, gradients, and patterns are rendered @2x.

R=pdr@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23482007

git-svn-id: svn://svn.chromium.org/blink/trunk@157008 bbb929c8-8fbe-4397-9dbb-9b2b20218538