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This moves the implementation of PageAlmostIdle from
PageSignal(Generator|Receiver|Observer), to a new "decorator" that
runs directly in the graph.

BUG=931407

Change-Id: I1e8c86e34f4cdd224cb3b77c7545debcffc820a4
Reviewed-on: https://chromium-review.googlesource.com/c/1471750
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635589}