[Dice] Split promo user actions

The NewAccount actions and histograms are split into 3 subcases:
- NewAccountPreDice: used on desktop pre-Dice
- NewAccountNoExistingAccount: adding an account when Chrome has no account
- NewAccountExistingAccount: adding an account when Chrome already has account(s).

Bug: 859142, 855754
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ifcf077341b00c70d07fcb8d2cf863384bb9b985e
Reviewed-on: https://chromium-review.googlesource.com/1133388
Commit-Queue: David Roger <[email protected]>
Reviewed-by: Ilya Sherman <[email protected]>
Reviewed-by: Boris Sazonov <[email protected]>
Reviewed-by: Mihai Sardarescu <[email protected]>
Reviewed-by: Jérôme Lebel <[email protected]>
Cr-Commit-Position: refs/heads/master@{#582136}
13 files changed