Keeping distinct platform runtime files takes up a lot of space. It also adds complexity and
greater possibility for mistakes, if a file for some arbitrary platform (like pgo) doesn't
exist, there will be a failure. This patch removes all platform specific runtime files and
replaces them with a single per suite file. The times in this file are averaged across all
platforms the tests run on. With the added space savings, we take into account the 50th
percentile of tests, instead of the 90th.
With --chunk-by-runtime enabled, test runtime data collected from automation is used to try and make all chunks take the same amount of time. So far only data for mochitest browser-chrome is added.
