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.

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,asapersson@webrtc.org,mflodman@webrtc.org,philipel@webrtc.org,terelius@webrtc.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:4914

[philipel1, 2015-11-10 14:57:43]

Created Reland of Work on flexible mode and screen sharing.

[commit-bot: I haz the power, 2015-11-10 14:57:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1432553008/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1432553008/1

[commit-bot: I haz the power, 2015-11-10 14:57:52]

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

[commit-bot: I haz the power, 2015-11-10 14:57:53]

No L-G-T-M from a valid reviewer yet. Only full committers are accepted.
Even if an L-G-T-M may have been provided, it was from a non-committer,
_not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[philipel, 2015-11-10 15:08:29]

The CQ bit was checked by philipel@webrtc.org

[commit-bot: I haz the power, 2015-11-10 15:08:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1432553008/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1432553008/1

[commit-bot: I haz the power, 2015-11-10 15:08:46]

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

[commit-bot: I haz the power, 2015-11-10 15:08:47]

No L-G-T-M from a valid reviewer yet. Only full committers are accepted.
Even if an L-G-T-M may have been provided, it was from a non-committer,
_not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.