login: Make flags filter compatible with Chrome logic. Chrome treats "--" the same as "-" as command flag prefix. Modify the flag filter so that it works as expected. BUG=b:36067509 TEST=Manual test Change-Id: I1532fd2da8f253991cdc84304098e14b09a579ea Reviewed-on: https://chromium-review.googlesource.com/509968 Commit-Ready: Long Cheng <[email protected]> Tested-by: Long Cheng <[email protected]> Reviewed-by: Dan Erat <[email protected]> (cherry picked from commit 911d00ce79d6bd2a5ede15f26c6a3de67c7c9992) Reviewed-on: https://chromium-review.googlesource.com/514427 Commit-Queue: Long Cheng <[email protected]> Trybot-Ready: Long Cheng <[email protected]>