Remove net::Filter direct dependency on IsSupportedMimeType

Remove net::Filter direct dependency on IsSupportedMimeType

There is a widespread consensus that mime type checks don't belong
to net/, they should be moved to content/. But in order to be able
to do that, first we need to remove all direct dependencies on
mime type checks under net/. This CL will remove such a direct
dependency from net::Filter::FixupEncodingTypes. We'll introduce
a new NetworkDelegate::IsSupportedMimeType API instead and that
will be overridden by actual network delegate implementations
that care about this (ChromeNetworkDelegate and ShellNetworkDelegate)

BUG=318217

[servolk, 2015-02-24 00:23:56]

servolk@chromium.org changed reviewers:
+ bengr@chromium.org, dalecurtis@chromium.org, gunsch@chromium.org,
mmenke@chromium.org, rdsmith@chromium.org, rsleevi@chromium.org,
wolenetz@chromium.org

[servolk, 2015-02-24 00:24:36]

servolk@chromium.org changed reviewers:
+ chcunningham@chromium.org, lcwu@chromium.org

[Ryan Sleevi, 2015-02-24 00:50:46]

//net not lgtm, per email discussion.

The goal is not to rework this dependency in //net through NetworkDelegate.

The first attempt is to remove this code entirely.
Barring that, the next attempt is to move this behaviour that is calling it all
the way into //content where it belongs.