Remove a pointless assert that fails with overflowed values

It turns out that
roundedIntPoint(LayoutUnit::max()) != LayoutUNit::max()
so disable the assert in BackgroundImageGeometry that is really just
checking that we have pixel snapped the phase. We don't check any other
values for pixel snapping, so there's no reason to keep checking phase.

And there's no danger to having an unrounded value here. Nothing breaks.

R=fmalita@chromium.org
BUG=626745

Review-Url: https://codereview.chromium.org/2180433002
Cr-Commit-Position: refs/heads/master@{#407255}

third_party/WebKit/Source/core/paint/BackgroundImageGeometry.cpp

@@ -418,7 +418,6 @@ void BackgroundImageGeometry::calculate(const LayoutBoxModelObject& obj, const L
     m_destRect.intersect(paintRect);
 
     // Snap as-yet unsnapped values.
-    DCHECK(m_phase == LayoutPoint(roundedIntPoint(m_phase)));
     setDestRect(LayoutRect(pixelSnappedIntRect(m_destRect)));
 }