[Performance test] Support "description" for PerfTestRunner.run and PerfTestRunner...

[Performance test] Support "description" for PerfTestRunner.run and PerfTestRunner.runPerSecond
https://bugs.webkit.org/show_bug.cgi?id=86696

Reviewed by Ryosuke Niwa.

We want to add some description for each PerfTestRunner.run and
PerfTestRunner.runPerSecond, so that people can know what each
run is testing.

Output example:

$ ./Tools/Scripts/run-perf-tests Bindings/first-child.html
Running Bindings/first-child.html (1 of 1)
DESCRIPTION: Benchmark for DOM attributes that return a Node object.
RESULT Bindings: first-child= 788.359076534 runs/s
median= 797.508097751 runs/s, stdev= 19.0972905207 runs/s, min= 746.666666667 runs/s, max= 801.001251564 runs/s

PerformanceTests:

* resources/runner.js:
(PerfTestRunner.logStatistics):
(PerfTestRunner.printStatistics):
(PerfTestRunner.runPerSecond):

Tools:

* Scripts/webkitpy/performance_tests/perftest.py:
(PerfTest):
(PerfTest.parse_output):
(PerfTest.output_statistics):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@117397 268f45cc-cd09-0410-ab3c-d52691b4dbfc