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

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}
(cherry picked from commit 553cec8b3c449a02a69a1663f5fb077f3abefba2)

Committed: https://chromium.googlesource.com/chromium/src/+/24aaf508f0647139069ecc0f395a88b3ad25a9ea

[raymes, 2015-02-25 23:11:21]

Committed patchset #1 (id:1) manually as
24aaf508f0647139069ecc0f395a88b3ad25a9ea.