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Before this change, MojoApplicatioHost was activated only if some
browser-side modules requested it. There was no way for renderer
to activate it.

This CL let MojoApplicationHost always get activated. Now
renderer can request services it needs without its own activation flow.

R=darin@chromium.org,tsepez@chromium.org
TEST=none
BUG=none

Review URL: https://codereview.chromium.org/563373002

Cr-Commit-Position: refs/heads/master@{#294969}