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With this CL, PageNode::IsLoading() transitions to true when data is
received for a top-level navigation to a different document. It
transitions to false when the page reaches an "almost idle" state,
based on CPU and network quiescence, or after an absolute timeout.

This definition of "loading" is considered better to drive policies,
as it indicates when the page stops doing work after a navigation,
rather than when the page is loading resources (it may still have a
lot of Javascript to run to present useful content to the user).

Change-Id: I4fc5b277933133229fde7585d6710f63e6b8244d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015820
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738272}