Disable UKM in incognito mode.

Add explicit checks to make sure UKM is disabled in incognito mode,
and add browser tests to check that behavior.

Reland of https://chromium-review.googlesource.com/537774 with changes
in the browser test to explicitly enable UKM Feature for the test, so
that it works properly on all bots.

TBR=rkaplow,lpromero

Bug: 734091
Change-Id: I22371a9730aab1140dd9dae2cabb9c4760699083
Reviewed-on: https://chromium-review.googlesource.com/548998
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Louis Romero <lpromero@chromium.org>
Reviewed-by: Martin Šrámek <msramek@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#482379}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 30f9a63797e1403c229b6e54a773339593f9c017
2 files changed