Separate xvfb.py logic into its own script

tools/isolate/isolate.py

Index: tools/isolate/isolate.py
diff --git a/tools/isolate/isolate.py b/tools/isolate/isolate.py
index d70296add3c96b0926751a239187b614aa204479..ad106b94598bc705b5d4fa66212a091671305936 100755
--- a/tools/isolate/isolate.py
+++ b/tools/isolate/isolate.py
@@ -121,14 +121,10 @@ def run(outdir, resultfile, indir, infiles, read_only, cmd):
     if read_only:
       tree_creator.make_writable(outdir, True)
 
-    # TODO(maruel): Remove me. Layering violation. Used by
-    # base/base_paths_linux.cc
-    env = os.environ.copy()
-    env['CR_SOURCE_ROOT'] = outdir.encode()
     # Rebase the command to the right path.
     cmd[0] = os.path.join(outdir, cmd[0])
     logging.info('Running %s' % cmd)
-    result = subprocess.call(cmd, cwd=outdir, env=env)
+    result = subprocess.call(cmd, cwd=outdir)
     if not result and resultfile:
       # Signal the build tool that the test succeeded.
       touch(resultfile)