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In addition to the normal separation of MojoMain() into its own build
target, this patch also renames a class and moves a class into the
surfaces:: namespace. This was done because they were non-unique
symbol names in the resultant binary.

(There were two classes named mojo::ContextProviderMojo and had
different implementations.)

BUG=477435

Review URL: https://codereview.chromium.org/1138293002

Cr-Commit-Position: refs/heads/master@{#329444}