Fixes to build WebRTC for Fuchsia

1. Added WEBRTC_FUCHSIA define.
2. Added PlatformThreadId typedef for Fuchsia.
3. Updated ifdefs for _strnicmp()/strncasecmd(), so _strnicmp()
   is used on all platforms
3. Updated ifdefs in clock.cc to avoid invalid assumption that
   POSIX = LINUX || MAC .

Bug: chromium:750940
Change-Id: Id7aa98e017f467bcebb78a0b298ba91655502072
Reviewed-on: https://webrtc-review.googlesource.com/31641
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21233}
diff --git a/BUILD.gn b/BUILD.gn
index cfa24a1..6153e4f 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -134,6 +134,9 @@
   if (is_mac) {
     defines += [ "WEBRTC_MAC" ]
   }
+  if (is_fuchsia) {
+    defines += [ "WEBRTC_FUCHSIA" ]
+  }
   if (is_win) {
     defines += [
       "WEBRTC_WIN",