MacViews: Implement a Menu style to match Yosemite

Menus in MacViews currently look like Mavericks. Yosemite menus are
darker, more transparent and blur the background. The first two are
straightforward. To blur, use CGSSetWindowBackgroundBlurRadius(), which
is also present in 10.6. However, the new style will only be used when
running on Yosemite or later.

When stepping up the transparency on the canvas background, subpixel AA
became a mess. For a nice fix, use a solid canvas background but make
the CALayer hosting the compositor translucent instead.

BUG=522365

Review URL: https://codereview.chromium.org/1314113003

Cr-Commit-Position: refs/heads/master@{#345820}