Fix an issue that download filename from content disposition is not sanitized

      The filename from content disposition may not be sanitized.
This change uses net::GetSuggestedFileName() to sanitize the filename
before passing it to java.
Also, there is no need to call URLUtil.guessFileName() since
GetSuggestedFileName() will always return a non-null value.

BUG=570750

Review URL: https://codereview.chromium.org/1717783002

Cr-Commit-Position: refs/heads/master@{#377447}