Fix flakiness dashboard by not converting the popup index to a string

This patch reverts a small change in [1] which caused the index
parameter to showPopupForBuild to be passed as a string. The fix in
[1] did need to convert the revision argument to be quoted, but not
the index argument. Due to how strings work in javascript
("3" + 1 = "31"), this mistake caused us to get an incorrect index
for (index + 1) which caused NaNs to show up in the popups. This
patch reverts the mistake and updates the tests appropriately. In
addition, I tested various failing ranges with live data and things
look correct with this change.

[1] https://src.chromium.org/viewvc/blink?view=rev&revision=178588

BUG=397015
NOTRY=true

Review URL: https://codereview.chromium.org/415163002

git-svn-id: svn://svn.chromium.org/blink/trunk@178854 bbb929c8-8fbe-4397-9dbb-9b2b20218538