Add |is_encrypted| in mojo AudioDecoderConfig.

Add |is_encrypted| in mojo AudioDecoderConfig.

We need it to supported encrypted media playback. Note that |is_encrypted| is
already in VideoDecoderConfig.

BUG=432998
TEST=Added unittest to cover this.
R=jrummell@chromium.org

Committed: https://crrev.com/f6c45017858d62e8e0ac1ce8a1c45bfe59b17b8e
Cr-Commit-Position: refs/heads/master@{#309618}

[jrummell, 2014-12-23 23:29:27]

lgtm

[xhwang, 2014-12-23 23:39:37]

The CQ bit was checked by xhwang@chromium.org

[commit-bot: I haz the power, 2014-12-23 23:40:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/820103002/1

[commit-bot: I haz the power, 2014-12-23 23:49:03]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-12-23 23:49:04]

Try jobs failed on following builders:
  android_chromium_gn_compile_rel on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/android_chromiu...)

[xhwang, 2014-12-24 04:39:33]

The CQ bit was checked by xhwang@chromium.org

[commit-bot: I haz the power, 2014-12-24 04:40:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/820103002/1

[commit-bot: I haz the power, 2014-12-24 05:23:37]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-12-24 05:24:41]

Patchset 1 (id:??) landed as
https://crrev.com/f6c45017858d62e8e0ac1ce8a1c45bfe59b17b8e
Cr-Commit-Position: refs/heads/master@{#309618}