Take out force browser close.

Take out force browser close.
Now that the tests are more stable with the batch runner, we won't force all
browser windows to close. If it seems this is still needed on the buildbots,
we'll move the logic into builbot_annotated_steps.py.
Committed: https://code.google.com/p/dart/source/detail?r=5263

[Emily Fortuna, 2012-03-09 00:29:12]

Hi Bill,
I'm sorry it's taken me so long to integrate your comments from this CL:
https://chromiumcodereview.appspot.com/9369051/ into our code, but hopefully now
that selenium tests are more stable with the batch runner, the force browser
close shouldn't be needed.

[Bill Hesse, 2012-03-09 09:37:02]

LGTM, with comments.

https://chromiumcodereview.appspot.com/9649011/diff/2001/tools/testing/dart/t...
File tools/testing/dart/test_runner.dart (right):

https://chromiumcodereview.appspot.com/9649011/diff/2001/tools/testing/dart/t...
tools/testing/dart/test_runner.dart:465: bool allowRetries = false;
false is always overwritten by null or the provided value in the constructor.

https://chromiumcodereview.appspot.com/9649011/diff/2001/tools/testing/dart/t...
tools/testing/dart/test_runner.dart:471: [this.allowRetries,
this.processQueue]);
this.allowRetries = false

https://chromiumcodereview.appspot.com/9649011/diff/2001/tools/testing/dart/t...
tools/testing/dart/test_runner.dart:487: if (allowRetries &&
testCase.usesWebDriver
According to the spec, the optional [this.allowRetries] parameter in the
constructor is effectively the same as [this.allowRetries = null], and it
overwrites the false default value.  I just checked in the vm and in dartboard,
and the value will be null.