Split GrContextThreadSafeProxy into its own files (take 2)

This just shuffles stuff around.

Change-Id: I90e24a73bf44dd3e88fc121a0528bf94e4fdead4
Reviewed-on: https://skia-review.googlesource.com/c/187786
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/src/gpu/GrDirectContext.cpp b/src/gpu/GrDirectContext.cpp
index c46d84e..9e6075c 100644
--- a/src/gpu/GrDirectContext.cpp
+++ b/src/gpu/GrDirectContext.cpp
@@ -9,6 +9,7 @@
 #include "GrContext.h"
 
 #include "GrContextPriv.h"
+#include "GrContextThreadSafeProxy.h"
 #include "GrGpu.h"
 
 #include "effects/GrSkSLFP.h"