[Android] Fix the test case about testExtensionBroadcast.

verifyResult() was called from sub and top frame, when it was
called from sub frame, messageReceivedInTop was null, then set
the title to "Fail". Title was got in java is "Fail", So this
test case faild.

The fix is to add a variable to check the function "verifyResult()"
was called from top or sub frome.

BUG=https://crosswalk-project.org/jira/browse/XWALK-530