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The mouse cursor element is positioned relative to the document body (ie, ignoring
the title bar for apps v2). However, its position was being set using the x and y
coordinates of the mouse event, which are relative to the client viewport. This CL
fixes this, using offsetX/Y, which are relative to the target element.

BUG=410473

Review URL: https://codereview.chromium.org/923013004

Cr-Commit-Position: refs/heads/master@{#317447}