Win: Trim menu separators to stay within the outer menu border
Win: Trim menu separators to stay within the outer menu border

Currently menu separators draw the full width of the menu. If there is a
menu border stroke, the separator will draw over it in a different
color.

To fix, if there's a menu border, inset the horizontal bounds of the
menu separator paint bounds by 1. (1 = the stroke width used by
RoundRectPainter::Paint()).

Desktop linux is also affected by this, but
NativeThemeGtk2::GetSystemGdkColor() uses the same colors for the border
and separator so there is no difference. (Windows: border=0xbababa
separator=0xe9e9e9).

BUG=524322
TEST=Visual, screenshots at http://crbug.com/524322

Review URL: https://codereview.chromium.org/1315783002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#345504}

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