Fix a crash when download a DRM file through context menu

This is a regression introduced by
https://codereview.chromium.org/1717783002.
Previously we use URLUtil.guessFileName() to generate file names for
download.
However, to fix crbug/570750, we rely on the native code to pass the
file name.
However, drm download through context menu bypasses the native download
code.
As a result, we need to generate the file name in Java if it is null.
Since content disposition is null, this will not cause the same problem
as reported in crbug/570750.

BUG=608258

Review-Url: https://codereview.chromium.org/2013643002
Cr-Commit-Position: refs/heads/master@{#395750}