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
Committed:
https://crrev.com/fc0257d91a4a619fce84d274681f2bbc7f1c92b7
Cr-Commit-Position: refs/heads/master@{#438922}