Mac: Use [NSArray firstObject] for [NSScreen screens]

Crash reports indicate that the array is at times empty. Use firstObject
instead of objectAtIndex:0, because firstObject will return nil instead
of raising an exception.

BUG=529723

Review URL: https://codereview.chromium.org/1380083005

Cr-Commit-Position: refs/heads/master@{#352915}
24 files changed