Flickable's widthRatio and heightRatio are wrong when the content is smaller than the view and has topMargin/leftMargin

UPSTREAM BUG: https://bugreports.qt.io/browse/QTBUG-53726

Description (copied from the upstream bug I created)

According to Flickable's documentation, widthRatio and heightRatio define
"the percentage of the full view currently visible, scaled to 0.0 - 1.0".
At the moment, defining leftMargin so that
leftMargin+contentWidth < flickable.width
causes widthRatio to be wrongly evaluated to something != 1, whereas it should be 1, because Flickable's defaults at -leftMargin
(see https://github.com/qtproject/qtdeclarative/blob/5.6/src/quick/items/qquickflickable.cpp#L1592 ).
and when contentX is -leftMargin, the whole content fits inside the view.
As a consequence of that, in the current implementation you can also scroll the item from the testcase left and right, which shouldn't be possible (because item+margin are still smaller than the view).

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Additional Ubuntu-specific info:
This is currently blocking the correct implementation of the new scrollbars inside TextFields (ping kalikiana or me (faenil) on IRC for more info)

========TESTCASE============

import QtQuick 2.0

Flickable {
    id: flickable
    width: 200
    height: 200
    contentWidth: item.width
    contentHeight: item.height
    topMargin: 20
    leftMargin: 40
    Component.onCompleted: console.log("xPos", flickable.visibleArea.xPosition, "widthRatio", flickable.visibleArea.widthRatio)
    Connections {
        target: flickable.visibleArea
        onXPositionChanged: console.log("xPosChanged", flickable.visibleArea.xPosition)
        onWidthRatioChanged: console.log("widthRatioChanged", flickable.visibleArea.widthRatio)
    }
    Rectangle {
        id: item
        width: 100
        height: 100
        color: "black"
    }
}

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How to reproduce:
1) qmlscene testcase.qml
2) watch the console output

Actual result: widthRatio is != 1
Expected result: widthRatio == 1

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Fix: I worked on a fix which is being reviewed by upstream ---> https://codereview.qt-project.org/#/c/161043/