Revert of media: Simplify {Audio|Video}Decoder initialization callback.

Revert of media: Simplify {Audio|Video}Decoder initialization callback. (patchset #3 id:40001 of https://codereview.chromium.org/1156643007/)

Reason for revert:
Compile failure on mac: http://build.chromium.org/p/chromium/buildstatus?builder=Mac&number=3225

Original issue's description:
> media: Simplify {Audio|Video}Decoder initialization callback.
>
> Use base::Callback<void(bool)> instead of a PipelineStatusCB for decoder initialization where PipelineStatus is not really relevant. This helps clean up unnecessary uses of PipelineStatusCB so in the future we could plumb more info in PipelineStatusCB (e.g. error messages) to help debugging/testing.
>
> TBR=bbudge@chromium.org
> BUG=126070
> TEST=Updated unittests.
>
> Committed: https://crrev.com/cb8f0c2260d44d6610a4c157b70c01cda1f50913
> Cr-Commit-Position: refs/heads/master@{#333127}

TBR=watk@chromium.org,dalecurtis@chromium.org,xhwang@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=126070
Committed: https://crrev.com/e51969862119f6a721ec5299e41f01cd3df5fd4a
Cr-Commit-Position: refs/heads/master@{#333137}

[Nicolas Zea, 2015-06-05 21:27:31]

Created Revert of media: Simplify {Audio|Video}Decoder initialization callback.

[commit-bot: I haz the power, 2015-06-05 21:27:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1146913008/1

[commit-bot: I haz the power, 2015-06-05 21:30:26]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-05 21:31:17]

Patchset 1 (id:??) landed as
https://crrev.com/e51969862119f6a721ec5299e41f01cd3df5fd4a
Cr-Commit-Position: refs/heads/master@{#333137}