These files contain runtimes for test manifests in the tree. They are of the form:
{ '<path to manifest>': <average runtime in seconds> }
They are being used to normalize chunk durations so all chunks take roughly the same length of time.
The writeruntimes script can be used to generate this file:
$ ./writeruntimes
It will take awhile. You can optionally specify platforms or suites on the command line, but these should only be used for debugging purposes (not for committing an update to the data). For more info, see:
$ ./writeruntimes -- --help