Fix scale applied to drag images when rendering with HiDPI

This patch fixes a regression that was introduced by r163572, which refactored
ImageBuffer. As a part of the refactor, resolutionScale was removed from
ImageBuffer, causing NativeImageSkia (constructed by ImageBuffer::copyImage)
to no longer correctly transport the resolution scale information. To avoid
further confusion, this change removes resolutionScale from NativeImageSkia.
The only call site that needed to extract resolution scale from NativeImageSkia
was in DragImage, and it turns out that it is easy to access the page's
resolution scale at all the call sites where DragImages are created with
a device scale baked in, so there was no need to transport resolution scale
information in NativeImageSkia in the first place.

BUG=330976
TEST=blink manual test chromium/drag-image-accounts-for-device-scale.html

Review URL: https://codereview.chromium.org/143633002

git-svn-id: svn://svn.chromium.org/blink/trunk@165463 bbb929c8-8fbe-4397-9dbb-9b2b20218538