Add a media scanner convenience class.

Add a media scanner convenience class.

Create a new class that takes in a list of directories, recursively scans them for image and video files, and returns the resulting list.

BUG=420680
TEST=browser_test: FileManagerJsTest.MediaScannerTest

Committed: https://crrev.com/340e607f73159216a2e7359c0a71170b9d4668f9
Cr-Commit-Position: refs/heads/master@{#304582}

[Ben Kwa, 2014-11-18 05:51:41]

kenobi@chromium.org changed reviewers:
+ fukino@chromium.org

[fukino, 2014-11-18 06:56:25]

lgtm!

[Ben Kwa, 2014-11-18 08:46:06]

The CQ bit was checked by kenobi@chromium.org

[commit-bot: I haz the power, 2014-11-18 08:46:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/730353002/1

[commit-bot: I haz the power, 2014-11-18 10:25:29]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-11-18 10:26:13]

Patchset 1 (id:??) landed as
https://crrev.com/340e607f73159216a2e7359c0a71170b9d4668f9
Cr-Commit-Position: refs/heads/master@{#304582}

[Michael Achenbach, 2014-11-18 12:26:20]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/735013002/ by machenbach@chromium.org.

The reason for reverting is: [Sheriff] Fails on cros and ozone:
http://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%2...
http://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%2....