Update Tray Separator Opacity

The tray separator has an opacity of 0.24 and it looks too dim when the
shelf is completely opaque. Update the tray separator's opacity to 0.3.

BUG=641453

Review-Url: https://codereview.chromium.org/2281173002
Cr-Commit-Position: refs/heads/master@{#414798}

ash/common/system/tray/tray_constants.cc

@@ -89,7 +89,7 @@ const int kMenuIconSize = 20;
 const SkColor kMenuIconColor = SkColorSetRGB(0x5A, 0x5A, 0x5A);
 
 const int kHitRegionPadding = 4;
-const SkColor kSeparatorColor = SkColorSetA(SK_ColorWHITE, 0x3D);
+const SkColor kSeparatorColor = SkColorSetA(SK_ColorWHITE, 0x4D);
 const int kSeparatorWidth = 1;
 
 int GetTrayConstant(TrayConstant constant) {