cr: Allow passing command line arguments via the run command

This patch makes it possible to pass in command line arguments to the
run commmand, e.g.:

  $ cr run chrome https://www.youtube.com

The trade-off is that it is no longer possible to run multiple targets
using a single command.

Review URL: https://codereview.chromium.org/1638793002

Cr-Commit-Position: refs/heads/master@{#371506}

tools/cr/cr/commands/run.py

@@ -25,7 +25,7 @@ class RunCommand(cr.Command):
     cr.Builder.AddArguments(self, parser)
     cr.Installer.AddArguments(self, parser)
     cr.Runner.AddArguments(self, parser)
-    cr.Target.AddArguments(self, parser, allow_multiple=True)
+    cr.Target.AddArguments(self, parser, allow_multiple=False)
     self.ConsumeArgs(parser, 'the binary')
     return parser