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This adds counters for key, mouse, and touch events to
TabActivityWatcher::WebContentsData. When a tab is logged, these
counters are reported in the UKM. The counters are reset on each
navigation.

These counters are passed to TabSnapshotLogger in a new hierarchical
structure, which will be built upon to add browser-, tab- and page-
specific metrics. Separating these event counts into a separate struct
makes it easier to reset them when a navigation occurs.

This CL counts the absolute number of events -- as an improvement, we
would prefer to count user actions rather than, say, the exact
number of mouse-move events fired.

A unit test simulates mouse activity, but key and touch are harder to
pipe in from a unit test.

Bug: 784639
Change-Id: I693c34f0845c3fdeca6c1fb172e7347f9c2a504f
Reviewed-on: https://chromium-review.googlesource.com/794592
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520674}