Auto merge of #600 - gterzian:keep_scrolling_layer, r=glennw
Auto merge of #600 - gterzian:keep_scrolling_layer, r=glennw

Keeping the currently scrolling layer, unless new gesture starts

"If you scroll a long page, and somewhere on that page is a smaller scrollable element, you don't want that element to swallow the rest of your scroll gesture if it suddenly happens to move under your mouse. You want the current scroll gesture to keep scrolling the outer page." servo/servo#13249 (comment)

Page used for testing can be found online [here](https://samuknet.github.io/test_cases/nestedScroll/).

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