Propogate the tab foreground, background change events to data use

This CL observes WebContents visibility changes to record if a data use
entity (Chromium tab) is in foreground vs background. Further CLs will
record the histograms.

BUG=673820

Review-Url: https://codereview.chromium.org/2572823002
Cr-Commit-Position: refs/heads/master@{#438922}
9 files changed