Throttle browser button counter updates

[chrmod]

Counter updates can happen very fast and every update issues a call to chrome process. This change should improve performance - specifically for underpowered devices like mobiles.

[christophertino]

@IAmThePan do you remember what issue we had with passing tabID to button.update?

[chrmod]

btw. we've tested this change on mobile browser and seems to work without any issues. Would be probably better to have an update queue as there are multiple sources of "badge" updates.

[IAmThePan]

I don't quite remember but I think there were inconsistencies where the passed TabId wasn't for the currently active tab.
Examples:

• What if you have two windows open? Will it incorrectly set the badge count to the other window's active tab count?
• What if a tab closes and a Errored request is a tracker, will that incorrectly update the currently active tab.
• What if the TabId changes (a prefetched tab becomes active)?

[chrmod]

regardless of correct behavior of button.update this PR should introduce no functional change other than throttling. Note the _throttleButtonUpdate called without an argument will pass undefined value as tabId, which should be no different from passing no argument.