Profile: repurposed kTresholdPercent

Previously kTresholdPercent was the percentage of samples of the total
that a method must comprise before compiling.

I changed it to mean the threshold for a running total...i.e. compile
all the methods that comprise K% of the samples cumulatively.

(in the process fixed ProfileData#percent doc and changed its name)

Bug: 12877748
Change-Id: Ib0e18e525a16c11b189afc3d840c09183ac629de