Drop a few tab switch metrics.

Drop a few tab switch metrics.

This patch drops definitions of three metrics recorded when we bring a
tab back to foreground. We dropped them on Android and these were never
implemented on other platforms:

- Tab.SwitchedToForegroundLaunchedWithURL
- Tab.SwitchedToForegroundRevisit
- Tab.SwitchedToForgroundNumTabs

Being collected on tab switch, these do not actually tell us anything
about when tab switches are likely to occur, because they do not cover
the complementary case of tabs *not* being switched. To the best of my
knowledge, we haven't got any actionable results from these nor do we
track them at all.

BUG=None

Review URL: https://codereview.chromium.org/235023002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@263654 0039d316-1c4b-4281-b951-d872f2087c98