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Today, PageNode::IsLoading() transitions to "true" when a response is
received for a main frame different-document navigation
(WebContentsObserver::DidReceiveResponse()). It transitions to "false"
when the load has stopped (WebContentsObserver::DidStopLoading()) and
the page has been idling for a short timeout. This implies that
PageNode::IsLoading() is "false" between when a navigation starts and
when the initial response is received.

We want to build a policy that preempts BEST_EFFORT tasks when any
page is loading. If we use PageNode::IsLoading() to build that policy,
BEST_EFFORT tasks will still be able to execute between when a
navigation starts and when the initial response is received. This is
undesirable, because BEST_EFFORT tasks could still interfere with
navigation.

This CL solves the issue by replacing PageNode::IsLoading() with
PageNode::GetLoadingState(). This method returns an enum:

- kLoadingNotStarted: No top-level document has started loading yet.
- kLoading: A different top-level document is loading. This correspond
to WebContents::IsLoadingToDifferentDocument().
- kLoadedBusy: A different top-level document finished loading, but the
page did not  reach CPU and network quiescence since then.
- kLoadedIdle: The page reached CPU and network quiescence after loading
the current top-level document, or the load failed.

In a different CL, we will preempt BEST_EFFORT tasks when any page
is kLoading or kLoadedBusy.

Bug: 887407
Change-Id: Ie88a56457309aa86221f1ea31f7ceda731547b07
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2300400
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Joe Mason <joenotcharles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832480}