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Use TYPE_DEFAULT message-loop for browser-tests, since that's what
mojo_runner uses by default. We would normally use TYPE_DEFAULT
message-loops for chrome --mash too, but keep using TYPE_UI for now,
since mojo:ui needs that, and it is not known whether mojo:ui will be
run at the type of the message-loop creation.

Also, remove references to MessagePumpMojo since that has been removed.

BUG=none
TBR=jam@ for base/threading/

Review-Url: https://codereview.chromium.org/2351603002
Cr-Commit-Position: refs/heads/master@{#419486}