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.

This is a re-do of 730353002, which fixes a flakey test.

BUG=420680
TEST=browser_test: FileManagerJsTest.MediaScannerTest

Committed: https://crrev.com/24c3d02f6f904b623869fee4a20c688dc1695af5
Cr-Commit-Position: refs/heads/master@{#304971}

[Ben Kwa, 2014-11-20 01:57:36]

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

[fukino, 2014-11-20 02:01:27]

On 2014/11/20 01:57:46, Ben Kwa wrote:

lgtm

[fukino, 2014-11-20 02:01:27]

On 2014/11/20 01:57:46, Ben Kwa wrote:

lgtm

[Ben Kwa, 2014-11-20 02:04:44]

The CQ bit was checked by kenobi@chromium.org

[commit-bot: I haz the power, 2014-11-20 02:06:33]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/739263003/40001

[commit-bot: I haz the power, 2014-11-20 04:27:03]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2014-11-20 04:27:50]

Patchset 3 (id:??) landed as
https://crrev.com/24c3d02f6f904b623869fee4a20c688dc1695af5
Cr-Commit-Position: refs/heads/master@{#304971}