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This follows the pattern of NotificationPermissionClient.

This is part of a series of changes in both Blink and Chromium.

BUG=389194

TEST=PushMessagingBrowserTest*
TEST=LayoutTests/push_messaging/*

Review URL: https://codereview.chromium.org/707833002

git-svn-id: svn://svn.chromium.org/blink/trunk@184923 bbb929c8-8fbe-4397-9dbb-9b2b20218538