Reland of Decoupling rtc_base from Obj-C code (patchset #1 id:1 of https://codereview.webrtc.org/3008103002/ )
Reland of Decoupling rtc_base from Obj-C code (patchset #1 id:1 of https://codereview.webrtc.org/3008103002/ )

Reason for revert:
Trying to reland after fixing downstream project.

Original issue's description:
> Revert of Decoupling rtc_base from Obj-C code (patchset #8 id:140001 of https://codereview.webrtc.org/3001623003/ )
>
> Reason for revert:
> It breaks a downstream project.
>
> Original issue's description:
> > Decoupling rtc_base from Obj-C code
> >
> > The goal of this CL is to separate Obj-C/Obj-C++ code from targets which have
> > also C++ code (see https://bugs.chromium.org/p/webrtc/issues/detail?id=7743
> > for more information).
> >
> > To achieve this we have created 2 targets (rtc_base_objc and rtc_base_generic)
> > and rtc_base will act as a proxy between these targets (this way we can avoid a
> > circular dependency between rtc_base_generic and rtc_base_objc).
> >
> > BUG=webrtc:7743
> >
> > Review-Url: https://codereview.webrtc.org/3001623003
> > Cr-Commit-Position: refs/heads/master@{#19661}
> > Committed: https://chromium.googlesource.com/external/webrtc/+/a0a5b98937de0f0bf90e17011d2cbc8c632cb409
>
> TBR=kjellander@webrtc.org,kwiberg@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:7743
>
> Review-Url: https://codereview.webrtc.org/3008103002
> Cr-Commit-Position: refs/heads/master@{#19664}
> Committed: https://chromium.googlesource.com/external/webrtc/+/791a8f611da8cc6b0668ccc03bf7525e6ba72a46

TBR=kjellander@webrtc.org,kwiberg@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7743

Review-Url: https://codereview.webrtc.org/3004373002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#19681}

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