Reland "Remove criticalsection.cc dependency on platform_thread.cc."

This is a reland of 5af97ee3ad36cb6d386cfefa8c89d7c178015a07.

What's changed from the original?
- Moved the #include for <process.h> for Fuchsia to the types header.

Original change's description:
> Remove criticalsection.cc dependency on platform_thread.cc.
>
> As part of this, I'm moving global thread related functions over to
> platform_thread_types.* and introducing platform_thread_types.cc
> for the implementation.
>
> Change-Id: I4624877fb379e77d215f4ecd60f20b06d8df3ce5
> Bug: webrtc:8893
> Reviewed-on: https://webrtc-review.googlesource.com/53940
> Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
> Commit-Queue: Tommi <tommi@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#22037}

Bug: webrtc:8893
Change-Id: Idd0baa6756efd10ad11a5c6e4791eaa7a9dbc97f
Tbr: danilchap@webrtc.org
Reviewed-on: https://webrtc-review.googlesource.com/54800
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22068}
diff --git a/rtc_base/criticalsection.cc b/rtc_base/criticalsection.cc
index d92b478..f9168d8 100644
--- a/rtc_base/criticalsection.cc
+++ b/rtc_base/criticalsection.cc
@@ -11,7 +11,7 @@
 #include "rtc_base/criticalsection.h"
 
 #include "rtc_base/checks.h"
-#include "rtc_base/platform_thread.h"
+#include "rtc_base/platform_thread_types.h"
 
 // TODO(tommi): Split this file up to per-platform implementation files.