GitLab

It is equivalent to checking the result of GetNativeDrawingContext(), so
do that instead, in NativeThemeWin::Paint().

Also remove an unused param of NativeThemeWin::PaintIndirect().

BUG=675977
R=reed@google.com

Review-Url: https://codereview.chromium.org/2610003002
Cr-Commit-Position: refs/heads/master@{#441388}