For "Flat times" output, adds a column with cumulative percentage.  Also, since these percentages add up to less than 100% due to timer overhead, we go ahead and scale up all of the "Flat times" percentages proportionally, to make them add to 100%.  This makes it easier to compare breakdowns across different pexes.

Also, simplify the output to be simple tabular without extra brackets and whatnot.  E.g.:

Total across all functions - Cumulative times:
     Seconds   Pct  EventCnt TimerPath
    8.580407 82.6%         0 szmain
    8.580054 82.6%         0 szmain.parseModule
    8.405447 80.9%     46473 szmain.parseModule.parseFunctions
...
Total across all functions - Flat times:
     Seconds   Pct CumPct  EventCnt TimerName
    1.354678 16.0%  16.0%    100716 linearScan
    1.131292 13.3%  29.3%    139419 liveness
    0.728727  8.6%  37.9%     46473 genCode
...

BUG= none
R=jpp@chromium.org, kschimpf@google.com

Review URL: https://codereview.chromium.org/1870073005 .