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This adds UseCounter for local storage being first used before fcp and
after fcp. For prerendering, we expect to cancel the prerender on a
local storage access, because it uses a synchronous IPC. Note that while
getAll() is the only sync method in blink::mojom::StorageArea, any
initial use of local storage uses that to populate Blink's in memory
cache.

This adds a page load metrics observer which logs to Blink UseCounter on
Local Storage access. This is somewhat an abuse of the UseCounter, as
it's a targeted use case, but the UseCounter has good properties like
defining what a page load is. This will eventually be replaced with
Prerendering cancellation metrics when Prerendering is actually
implemented.

Bug: 1126305
Change-Id: If719c21115481d93930101e95d6fe86701f30523
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2497965
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822078}