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Add mojoms and StructTraits so that a complete DisplayLayout can be
serialized. This includes DisplayPlacement and some enums. DisplayLayout
is serialized via unique_ptr since it is normally stored as a
unique_ptr.

This is required to pass the DisplayLayout from mus-ws to chrome for the
display options page.

BUG=686848

Review-Url: https://codereview.chromium.org/2661663002
Cr-Commit-Position: refs/heads/master@{#447990}