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Currently, the responsibility of the ARC auth code
control flow was distributed across ArcAuthService
ArcSupportHost and background.js etc. This CL removes
it from background.js, as a preparation to move all
of them into a manageable form.

Now, background.js has two types of messages. 1) Action,
which is sent from the native code, to do something in
extension, and 2) Event to notify to the native code
that something (e.g., user clieck etc.) is happend in the
extension

BUG=b/31079732
TEST=Ran on test device. Ran tests.
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2388763002
Cr-Commit-Position: refs/heads/master@{#422734}