Content settings for <audio> and <video>.

This change adds a content setting for blocking HTML5 media (<audio>,
<video>). The setting is modeled after the image blocking setting. The
setting is exposed in chrome://settings/content#contentSettings and as
the chrome.contentSettings.media extension API.

The icons used to indicate allowed / blocked HTML5 media in the address
bar are temporary (red-tinted versions of the image icons) and intended
to facilitate manual testing for the feature. They should be replaced
with better-designed icons before M38 comes out.

BUG=50132

[eseidel, 2014-04-10 05:28:10]

Did you mean to send this for review? (rietveld doesn't mail when you upload)

[pwnall-personal, 2014-04-10 11:25:07]

On 2014/04/10 05:28:10, eseidel wrote:
> Did you mean to send this for review? (rietveld doesn't mail when you upload)

I have the Blink side of the patch part under review, as
https://codereview.chromium.org/27694002/

I figured I should wait for the Blink review to settle before I send this to
someone.

[pwnall-personal, 2014-06-14 05:23:16]

I think I got this rebased, but I can't use the trybots because git-cl is
corrupting my images. I'll try to figure this out before sending the CL out for
review.

Infra bug filed: https://crbug.com/384658

[Bernhard Bauer, 2014-07-01 09:12:50]

Looks pretty good in general! I agree that we should get nicer images though,
and we'll also want to get sign-off on the strings.

https://codereview.chromium.org/27635002/diff/852001/chrome/app/generated_res...
File chrome/app/generated_resources.grd (right):

https://codereview.chromium.org/27635002/diff/852001/chrome/app/generated_res...
chrome/app/generated_resources.grd:354: +        Sounds and videos
We usually use the term "audio" instead of "sounds".

https://codereview.chromium.org/27635002/diff/852001/chrome/app/generated_res...
chrome/app/generated_resources.grd:7737: +        Media
We should probably decide whether we want to use the "media" or "audio/video".

https://codereview.chromium.org/27635002/diff/852001/chrome/browser/content_s...
File chrome/browser/content_settings/content_settings_policy_provider.cc
(right):

https://codereview.chromium.org/27635002/diff/852001/chrome/browser/content_s...
chrome/browser/content_settings/content_settings_policy_provider.cc:32:
prefs::kManagedDefaultMediaSetting,
Not all of these are immediately necessary. The changes in this file for example
are if we want enterprise policy support for these content settings (which is
something we don't have for all content settings). It might make sense to pull
these out into a separate CL that someone from the enterprise team can then look
at.

https://codereview.chromium.org/27635002/diff/852001/chrome/browser/prefs/pre...
File chrome/browser/prefs/pref_functional_browsertest.cc (right):

https://codereview.chromium.org/27635002/diff/852001/chrome/browser/prefs/pre...
chrome/browser/prefs/pref_functional_browsertest.cc:131:
EXPECT_TRUE(browser()->profile()->GetPrefs()->GetBoolean(
I would probably use ASSERT_TRUE, because if Javascript is not enabled, the rest
of the test will fail anyway.