Use SkMutex in SkSL.

Change-Id: I1b14e4c8de0ca60a0ebbdbc225befc5074e8de22
Bug: skia:10862
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/329177
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index eeaa984..f0231c7 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -558,6 +558,8 @@
     sources = [
       "src/core/SkMalloc.cpp",
       "src/core/SkMath.cpp",
+      "src/core/SkSemaphore.cpp",
+      "src/core/SkThreadID.cpp",
       "src/ports/SkMemory_malloc.cpp",
       "src/sksl/SkSLMain.cpp",
     ]