V8 Buildbot: Fix builder order using the default presorting.
V8 Buildbot: Fix builder order using the default presorting.

The builder prioritization of buildbot is inconvenient. CL
https://codereview.chromium.org/417533002/ introduced
ordering builders for V8. Unfortunately, all builders with
the same priority (the majority) are not sorted anymore with
the default buildbot sorter, but keep the order in which
they were added. Now some builders with equal priorities are 
leaking far behind.

This sorts the builders first with the same sorting function
buildbot is using (which takes the age of build requests
into account) and afterwards applies custom priorities.

See also: https://code.google.com/p/chromium/codesearch#chromium/tools/build/third_party/buildbot_8_4p1/buildbot/process/botmaster.py&l=485

BUG=397087

Review URL: https://codereview.chromium.org/413163002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@285343 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build