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Simplifies the code path for the Framebust block infobar that was
trying to share code with the desktop intrvention UI but ended up
being unused. Makes the implementation closer to the one for the
Near-OOM intervention.
Most importantly, exposes Accept and DeclineIntervention methods
for the Framebust Block intervention, that now enables callers
to react when the intervention is accepted.

Bug: 781890
Change-Id: I845f76bdbddf85f49c8e0fe8548a2e78d2e213b3
Reviewed-on: https://chromium-review.googlesource.com/779521
Commit-Queue: Nicolas Dossou-Gbété <dgn@chromium.org>
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518319}