Change horizontal/vertical separators in OSX bubble UI to be less light.

BUG=558839

Review URL: https://codereview.chromium.org/1457883004

Cr-Commit-Position: refs/heads/master@{#361167}

chrome/browser/ui/cocoa/base_bubble_controller.mm

@@ -210,7 +210,7 @@
   base::scoped_nsobject<NSBox> spacer([[NSBox alloc] initWithFrame:frame]);
   [spacer setBoxType:NSBoxSeparator];
   [spacer setBorderType:NSLineBorder];
-  [spacer setAlphaValue:0.2];
+  [spacer setAlphaValue:0.75];
   return [spacer.release() autorelease];
 }
 
@@ -219,7 +219,7 @@
   base::scoped_nsobject<NSBox> spacer([[NSBox alloc] initWithFrame:frame]);
   [spacer setBoxType:NSBoxSeparator];
   [spacer setBorderType:NSLineBorder];
-  [spacer setAlphaValue:0.2];
+  [spacer setAlphaValue:0.75];
   return [spacer.release() autorelease];
 }