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Currently, mojo resources are served from chrome://mojo and each mojo
WebUI controller replaces the WebUIDataSource for that origin with one
containing the resource it needs on construction. If a WebUI page's
controller (or an equivalent one) was not the last one to be
constructed, any requests for mojo bindings for its mojo interfaces
will fail. This change fixes this problem by serving the mojo resources
from the same origin as the WebUI itself.

BUG=557540

Review URL: https://codereview.chromium.org/1457623004

Cr-Commit-Position: refs/heads/master@{#361275}