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We are in the process of attributing as many mojo interfaces as possible
to specific execution contexts, which would enable better control by the
browser over those contexts. As part of that effort, this CL uses the
execution context's BrowserInterfaceBroker to bind the
PushMessaging remote and moves the code registering the binders
accordingly.

Bug: 1126758
Change-Id: Idfd27baf90fc2fa5e150f0d97753e21757104607
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2596587Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Tal Pressman <talp@google.com>
Cr-Commit-Position: refs/heads/master@{#841337}