Reland of Work on flexible mode and screen sharing.

Reland of Work on flexible mode and screen sharing. (patchset #1 id:1 of https://codereview.webrtc.org/1438543002/ )

Reason for revert:
Failed test not related to this CL (test fails on
master at an earlier date), re-landing original CL..

(This time from my @webrtc account.)

Original issue's description:
> Revert of Work on flexible mode and screen sharing. (patchset #28 id:520001 of https://codereview.webrtc.org/1328113004/ )
>
> Reason for revert:
> Seems to break VideoSendStreamTest.ReconfigureBitratesSetsEncoderBitratesCorrectly on Linux Memcheck buildbot.
>
> Original issue's description:
> > Work on flexible mode and screen sharing.
> >
> > Implement VP8 style screensharing but with spatial layers.
> > Implement flexible mode.
> >
> > Files from other patches:
> > generic_encoder.cc
> > layer_filtering_transport.cc
> >
> > BUG=webrtc:4914
> >
> > Committed: https://crrev.com/77ccfb4d16c148e61a316746bb5d9705e8b39f4a
> > Cr-Commit-Position: refs/heads/master@{#10572}
>
> TBR=sprang@webrtc.org,stefan@webrtc.org,philipel@google.com,asapersson@webrtc.org,mflodman@webrtc.org,philipel@webrtc.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:4914
>
> Committed: https://crrev.com/0be8f1d347bdb171462df89c2a4c69b3f3eb7519
> Cr-Commit-Position: refs/heads/master@{#10578}

TBR=sprang@webrtc.org,stefan@webrtc.org,philipel@google.com,asapersson@webrtc.org,mflodman@webrtc.org,terelius@webrtc.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:4914
Committed: https://crrev.com/cfc319be1d6afec77bd41eeb70d3e7886dd524db
Cr-Commit-Position: refs/heads/master@{#10581}

[philipel, 2015-11-10 15:17:07]

Created Reland of Work on flexible mode and screen sharing.

[commit-bot: I haz the power, 2015-11-10 15:17:18]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1431283002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1431283002/1

[commit-bot: I haz the power, 2015-11-10 15:17:27]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-10 15:17:37]

Patchset 1 (id:??) landed as
https://crrev.com/cfc319be1d6afec77bd41eeb70d3e7886dd524db
Cr-Commit-Position: refs/heads/master@{#10581}