Made top controls work with virtual viewport pinch-to-zoom. (Blink)

Source/core/frame/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 3012 matching lines @@
     }
 }
 
 void FrameView::setTopControlsViewportAdjustment(float adjustment)
 {
     m_topControlsViewportAdjustment = adjustment;
 }
 
 IntPoint FrameView::maximumScrollPosition() const
 {
-    FloatSize visibleContentSizeF = unscaledVisibleContentSize(ExcludeScrollbars);
-    visibleContentSizeF.expand(0, -m_topControlsViewportAdjustment);
-    visibleContentSizeF.scale(1 / visibleContentScaleFactor());
-    IntSize visibleSize = expandedIntSize(visibleContentSizeF);
+    // Make the same calculation as in CC's LayerImpl::MaxScrollOffset()
+    FloatSize visibleSize = unscaledVisibleContentSize(ExcludeScrollbars);
+    visibleSize.expand(0, m_topControlsViewportAdjustment);
 
-    IntPoint maximumOffset(
-        contentsWidth() - visibleSize.width() - scrollOrigin().x(),
-        contentsHeight() - visibleSize.height() - scrollOrigin().y());
-    maximumOffset.clampNegativeToZero();
-    return maximumOffset;
+    FloatSize contentBounds = contentsSize();
+    contentBounds.scale(visibleContentScaleFactor());
+    contentBounds = flooredIntSize(contentBounds);
+
+    FloatSize maximumOffset = contentBounds - visibleSize - toIntSize(scrollOrigin());
+
+    // Convert back to CSS pixels.
+    maximumOffset.scale(1 / visibleContentScaleFactor());
+
+    IntPoint snappedMaximumOffset = flooredIntPoint(maximumOffset);
+    snappedMaximumOffset.clampNegativeToZero();
+    return snappedMaximumOffset;
 }
 
 } // namespace blink