In CSSPaintDefinition::Paint, the zoom is calculated from StyleRef. Instead of doing that, we should probably passing two sizes in this function, where the current implementation has only one size which is in css pixels. We should be able to get the native size in the BoxPainter, and then the ratio between native size vs css pixels will be the backingStorePixelRatio.

It is decided here: https://github.com/w3c/css-houdini-drafts/issues/436, that we would only need to expose the device scale factor, and no need for sub pixel accumulation.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/429cd7c7bf7d4f1d94bc570312ce95a8e86268e5

commit 429cd7c7bf7d4f1d94bc570312ce95a8e86268e5
Author: Xida Chen <xidachen@chromium.org>
Date: Tue Aug 15 15:01:57 2017

Add devicePixelRatio to PaintWorkletGlobalScope

In this CL, we add the devicePixelRatio property to the
PaintWorkletGlobalScope.idl. This property could be useful for 
developers to draw perfect graphics on high DPI devices.

A test is added to ensure that this property has the same value as the
window.devicePixelRatio.

Bug:  728599 
Change-Id: Ia6179164989d4f564df06a87f1399adff19a0599
Reviewed-on: https://chromium-review.googlesource.com/602367
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#494379}
[modify] https://crrev.com/429cd7c7bf7d4f1d94bc570312ce95a8e86268e5/third_party/WebKit/LayoutTests/VirtualTestSuites
[add] https://crrev.com/429cd7c7bf7d4f1d94bc570312ce95a8e86268e5/third_party/WebKit/LayoutTests/external/wpt/css-paint-api/hidpi/device-pixel-ratio-ref.html
[add] https://crrev.com/429cd7c7bf7d4f1d94bc570312ce95a8e86268e5/third_party/WebKit/LayoutTests/external/wpt/css-paint-api/hidpi/device-pixel-ratio.html
[modify] https://crrev.com/429cd7c7bf7d4f1d94bc570312ce95a8e86268e5/third_party/WebKit/LayoutTests/http/tests/worklet/webexposed/global-interface-listing-paint-worklet-expected.txt
[add] https://crrev.com/429cd7c7bf7d4f1d94bc570312ce95a8e86268e5/third_party/WebKit/LayoutTests/virtual/scalefactor200/external/wpt/css-paint-api/hidpi/README.txt
[modify] https://crrev.com/429cd7c7bf7d4f1d94bc570312ce95a8e86268e5/third_party/WebKit/Source/modules/csspaint/PaintWorkletGlobalScope.cpp
[modify] https://crrev.com/429cd7c7bf7d4f1d94bc570312ce95a8e86268e5/third_party/WebKit/Source/modules/csspaint/PaintWorkletGlobalScope.h
[modify] https://crrev.com/429cd7c7bf7d4f1d94bc570312ce95a8e86268e5/third_party/WebKit/Source/modules/csspaint/PaintWorkletGlobalScope.idl