This patch remove the NativeDrawingContext from ui/gfx/native_widget_types.h,
renames the PlatformSurface in skia/ext/platform_surface.h to NativeDrawingContext,
and updates the clients of gfx::NativeDrawingContext to the skia one.

BUG=76105
TEST=skia_unittests,printing_unittests
R=danakj@chromium.org,thestig@chromium.org,fmalita@chromium.org
TBR=sky@chromium.org

Review-Url: https://codereview.chromium.org/2400873002
Cr-Commit-Position: refs/heads/master@{#424585}