Fix a couple of bugs in signature generation.

Missing comma messed up one of the regex pieces, and we want at
MOST 15 stack frames, not at LEAST.
Add some test cases for generating signatures
Aggregate chromehang stack info for dashboard.
Remove the "stack map" stuff
Combine stacks and get the top N.

Aggregate chromehang stack info and get the top MAX_ROWS by
frequency (number of pings), median duration, total duration.