Xorg+dummy: set resolution to first size in list
Xorg+dummy: set resolution to first size in list

Change the display resolution to the first size in the list before
starting the session.

BUG=596125

Review-Url: https://codereview.chromium.org/1980163003
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#394578}

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