Sets a white background color for iOS unittest windows.

This makes the UILabel with the test suite name visible again.

BUG=None
TEST=None

Review URL: https://codereview.chromium.org/637403003

Cr-Commit-Position: refs/heads/master@{#298872}

base/test/test_support_ios.mm

@@ -70,6 +70,7 @@ - (BOOL)application:(UIApplication *)application
 
   // Yes, this is leaked, it's just to make what's running visible.
   window_.reset([[UIWindow alloc] initWithFrame:bounds]);
+  [window_ setBackgroundColor:[UIColor whiteColor]];
   [window_ makeKeyAndVisible];
 
   // Add a label with the app name.