Eliminate defines in talk/

Replace LINUX, OSX and IOS defines with WEBRTC_ prefixed versions.
Remove no longer used defines from talk/build/common.gypi due to
previously migrated sources (into webrtc/p2p and webrtc/libjingle).

When this is rolled into Chromium, we can also clean up the platform
defines in
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/libjingle/libjingle.gyp

NOTRY=True
BUG=webrtc:5420
TESTED=Ran all compile trybots with --clobber flag.
TBR=tommi@webrtc.org

Review URL: https://codereview.webrtc.org/1588453005

Cr-Commit-Position: refs/heads/master@{#11254}

webrtc/base/unixfilesystem.cc

@@ -373,7 +373,7 @@ bool UnixFilesystem::GetAppPathname(Pathname* path) {
   return success;
 #elif defined(__native_client__)
   return false;
-#elif IOS
+#elif WEBRTC_IOS
   IOSAppName(path);
   return true;
 #else  // WEBRTC_MAC && !defined(WEBRTC_IOS)