[UseZoomForDSF] Convert draggable region from Viewport to Window coordinates
[UseZoomForDSF] Convert draggable region from Viewport to Window coordinates

These coordinates are currently the same but will differ when use-zoom-for-dsf is enabled. (See bug for discussion and design doc)

BUG=485650

Review URL: https://codereview.chromium.org/1602203007

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#370643}

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