Rename constants files in webrtc/{media,p2p}

Multiple sources with the same names forces ugly GYP hacks in
Chromium's libjingle.gyp. Rename the sources in WebRTC to
enable cleaning this up in Chromium.

To summarize:
webrtc/media/base/constants.{cc,h} -> mediaconstants.{cc,h}
webrtc/p2p/base/constants.{cc,h} -> p2pconstants.{cc,h}

This CL will require coordinating landing a roll in Chromium.

BUG=webrtc:4256
NOTRY=True

Review URL: https://codereview.webrtc.org/1750593002

Cr-Original-Commit-Position: refs/heads/master@{#11842}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f475277547c8ff48e4d7dd0007ab1bdb8cbb3b4a
34 files changed