Reland of Fixed the audio backgrounding bug

Reland of Fixed the audio backgrounding bug (patchset #1 id:1 of https://codereview.chromium.org/1383123003/ )

Reason for revert:
My assumption about that this CL caused WebRTC tests to fail was invalid.

Original issue's description:
> 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}

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/ab0349e64ed0c34a20bb321031bbcf9443cf924d
Cr-Commit-Position: refs/heads/master@{#352568}

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

Created Reland of Fixed the audio backgrounding bug

[commit-bot: I haz the power, 2015-10-06 08:42:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1386103002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1386103002/1

[commit-bot: I haz the power, 2015-10-06 08:43:36]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-06 08:44:30]

Patchset 1 (id:??) landed as
https://crrev.com/ab0349e64ed0c34a20bb321031bbcf9443cf924d
Cr-Commit-Position: refs/heads/master@{#352568}