Revert of Fixed the audio backgrounding bug

Revert of Fixed the audio backgrounding bug (patchset #28 id:1220001 of https://codereview.chromium.org/1214883004/ )

Reason for revert:
Speculative revert since we see failing tests in the WebRTC waterfall related to audio.

See

https://build.chromium.org/p/chromium.webrtc/builders/Win7%20Tester/builds/21950
https://build.chromium.org/p/chromium.webrtc/builders/Mac%20Tester/builds/43969

(after landing...)

Looked more into the details but I don't think this CL is the cause. Sorry. Will let it run one
round just in case and then revert the revert. 

Original issue's description:
> Fixed the audio backgrounding bug.
>
> Fixed the fact that we don't background a tab that initially skipped backgrounding because audio was playing (at the moment the tab was hidden) when the audio stops.
>
> We now make calls to UpdateProcessPriority when audio starts or stops.
>
> BUG=491895
>
> Committed: https://crrev.com/349188e9ed037b427815c9e5ad55223d043ab6fd
> Cr-Commit-Position: refs/heads/master@{#352421}

TBR=gab@chromium.org,dalecurtis@chromium.org,nick@chromium.org,miu@chromium.org,avi@chromium.org,rsesek@chromium.org,jam@chromium.org,sebsg@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=491895
Committed: https://crrev.com/4148513e2c1d40f3206396f8fff62f23b870459b
Cr-Commit-Position: refs/heads/master@{#352562}

[henrika (OOO until Aug 14), 2015-10-06 07:38:27]

Created Revert of Fixed the audio backgrounding bug

[commit-bot: I haz the power, 2015-10-06 07:39:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1383123003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1383123003/1

[commit-bot: I haz the power, 2015-10-06 07:40:31]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-06 07:41:08]

Patchset 1 (id:??) landed as
https://crrev.com/4148513e2c1d40f3206396f8fff62f23b870459b
Cr-Commit-Position: refs/heads/master@{#352562}

[henrika (OOO until Aug 14), 2015-10-06 08:41:05]

I was wrong. Will revert the revert.

[henrika (OOO until Aug 14), 2015-10-06 08:42:26]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1386103002/ by henrika@chromium.org.

The reason for reverting is: My assumption about that this CL caused WebRTC
tests to fail was invalid..