Ensure streams aren't intercepted unless their associated plugin is enabled

Previously streams would be intercepted even if the MimeHandlerView plugin
associated with them was disabled which would prevent the resource
from being download if a plugin wasn't being enabled to handle it.

BUG=459383

Review URL: https://codereview.chromium.org/933093002

Cr-Commit-Position: refs/heads/master@{#317673}
1 file changed