Update drag images to paint from the nearest self painting layer

Before this patch there were two codepaths for painting drag images:
1) Boxes were drawn using paintAsPseudoStackingContext which would
fail to paint composited children (https://crbug.com/585611).
2) Everything else was drawn using paintIntoDragImage which paints
starting at the view so any transparent regions of the drag element
would be filled in with the content behind the drag element.

This patch unifies drag image rendering under a single approach where
painting begins at the nearest self painting layer to the dragged
element. This has two nice properties:
1) self painting children are painted.
2) transparency is supported as long as the drag element has a layer.

Neither Safari nor Firefox really support these usecases so this
approach is a best-effort at supporting common usecases without
invasive surgery on the paint code. Some behavior is different such
as not including the dragged element's transform.

As part of this patch, paintIntoDragImage has been merged into
dragImageForSelection which is now the only caller.

BUG=585611

Review URL: https://codereview.chromium.org/1736893002

Cr-Commit-Position: refs/heads/master@{#378021}

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