Remove DriverProxy._running_drivers from webkitpy.layout_tests.port

Remove DriverProxy._running_drivers from webkitpy.layout_tests.port
https://bugs.webkit.org/show_bug.cgi?id=116492

Reviewed by Dirk Pranke.

Merge https://chromium.googlesource.com/chromium/blink/+/1af2f0be5a1746784ff114952f29f7b8fa9895f3.

The DriverProxy class had the concept of keeping a pool of started DRTs around (indexed by command line)
so that we didn't necessarily need to restart DRT when the command line for a test changed (e.g., when
starting to run tests in a virtual suite). However, the pool was per-worker (so not shared across the machine)
and we didn't cap the number of processes started, so we would end up with a whole bunch of DRTs (3x-4x
the number of workers started). If anything, this seemed to make the tests slightly slower and probably added
to memory pressure on the machine.

This change removes the pool code; now we'll simply restart DRT when the command line changes. With this change,
we can actually fold the restarting code into Driver itself (and get rid of DriverProxy), but I'll leave that
for a subsequent patch just to make things easier to follow.

* Scripts/webkitpy/port/driver.py:
(DriverProxy.__init__):
(DriverProxy.run_test):
(DriverProxy.has_crashed):
(DriverProxy.stop):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@150416 268f45cc-cd09-0410-ab3c-d52691b4dbfc