Mac Picker UI: Screen Auto Selection

This CL improves usability of Desktop Capture Picker on Mac
a little.

1. we auto select the first screen source,
2. we omit the label "Entire Screen" in the case there is only one screen, as this is redundant.

BUG=580116,630172

Review-Url: https://codereview.chromium.org/2168853003
Cr-Commit-Position: refs/heads/master@{#407490}