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This CL adds a UKM which counts "pageloads" by type for each top-level.
document. This will be used to diagnose changes to the "total pageloads"
shown on stability dashboard.

UKM review:
https://docs.google.com/document/d/1Nd5sKBdsq_sBqZ03OaWVBEloo4RGA00k_msYatkiV_0/edit?usp=sharing

Bug: 1075553
Change-Id: Ieadcd6b92eee371a695ebf9af1c8f0bce371d68a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2508441
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Auto-Submit: François Doray <fdoray@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824567}