Performance improved in dmprof visualizer.

There are no common benchmarks are available, so I measured
performance with
 - Used actual dmprof output with 11 snapshots from a browser process.
 - Measured time to process reparse() method in Profiler.setSub() function.
 - Breakdown "mmap-tcmalloc" with "browser-module" subs by hand.

Before: 3700 ms
After : 1100 ms


BUG=259206

Review URL: https://codereview.chromium.org/36653002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230425 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230425