Page load performance followups

[shinglyu]

In order not to spam the issue tracker, I'm collecting all the followup tasks for etc/ci/performance (i.e. the servo-perf runner) here. Most of them are pretty easy, so feel free to open a bug for each item and work on it.

Easy

• Add a expected timeout flag for our manifest
• Refactor the print() to python logging
• Discard the first 20% of test result to reduce noise
• Record profile when running tests
• Compare with chrome
• A commandline tool to compare the geometric mean of all testcases

Medium

• Automatic bisection
• Generate a Perfherder alert when there is a performance regression
• Update TP5 test cases (snapshot latest Alexa Top 500 websites as testcase)
• Parallelize the test run
• Record the memory usage during test run

Hard

• Debug running forever bug