The Feature Engagement Tracker provides a client-side backend for
displaying feature enlightenment or in-product help with a clean and
easy to use API to be consumed by the UI frontend. The backend behaves
as a black box and takes input about user behavior. Whenever the
frontend gives a trigger signal that in-product help could be displayed,
the backend will provide an answer to whether it is appropriate to show
it or not.

This CL adds a new component for this, with both a public C++ and a
public Java API. It also sets up the KeyedService factory in //chrome,
and hooks up the bridge between the C++ and Java implementations of the
backend. The Java version is a pass through, and all business logic is
implemented in C++. This means that callers do not have to care whether
they use the Java or C++ version.

It has been implemented as a component to ensure that other components
can use it directly, and to ensure that it would be easy to use the
same system across different platforms.

BUG=706309

Review-Url: https://codereview.chromium.org/2782113002
Cr-Commit-Position: refs/heads/master@{#461215}