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Reimplements parts of //third_party/libjingle's build in GN.

The new build is much less flexible than the original. No
configuration variables have been ported. Some windows-specific configs
have been left out and annotated with TODOs.  Most signifcantly, the
entirety of the 'if (enable_webrtc)' block has been omitted, because the
GN build for webrtc isn't working yet.

Despite these limitations, it is an adequate replacement for the
'stub' libjingle target in most situations.

Note, this CL is a near direct copy of rlarocque's work at
   https://codereview.chromium.org/455583002/

BUG=none
TBR=sky,pavely,rlarocque

Review URL: https://codereview.chromium.org/498603002

Cr-Commit-Position: refs/heads/master@{#291587}