Adding possibility to set usage of temporal layers from outside of VP8 gyp file.
Adding possibility to set usage of temporal layers from outside of VP8 gyp file.

Review URL: https://webrtc-codereview.appspot.com/926005

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: c0ceb8227fbefbde33e30f18e65b44bb2bd70d1b

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