Added the Ash.Tab.TimeBetweenSwitchToExistingTabUserActions histogram to track time between tab switches.

TEST=Manually verified histogram data added on ChromeOS.

BUG=478930

Review URL: https://codereview.chromium.org/1124153007

Cr-Commit-Position: refs/heads/master@{#330217}
9 files changed