[bisect] - Let perf try jobs specify revisions to test.
Previously the perf try bots always used ToT and built with/without patch. This CL lets you specify a before/after revision instead, so that we can kick off perf try jobs with traces from the dashboard. Also did some refactoring to break up _RunPerformanceTest a bit.
BUG=433456
TEST=
Revisions Specified: http://build.chromium.org/p/tryserver.chromium.perf/builders/linux_perf_bisect/builds/1088
Existing: http://build.chromium.org/p/tryserver.chromium.perf/builders/linux_perf_bisect/builds/1087
Review URL: https://codereview.chromium.org/
740633006
Cr-Commit-Position: refs/heads/master@{#306427}