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TabMetricsLogger logs the TabMetrics UKM event for a background tab.
This event includes the state of the tab, as well as some slightly
tricky or nullable fields.

Add a new TabFeatures struct that contains all this information, and
generate the struct in a separate function. The UKM logging function
now simply forwards info from the TabFeatures struct to the UKM event.

Later we will use the same TabFeatures to calculate tab scores with an
ML model. Encapsulating the logic for these features in a single
function ensures that we use the same logic for logging as we will for
inference.

Bug: 784639, 783989
Change-Id: I11e64119ce30465f98409f54f7e97af0251ce4b3
Reviewed-on: https://chromium-review.googlesource.com/1028562
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554694}