Add init-once to threaded backend

For the simplicity of this CL, I haven't enabled DAA for init-once yet.
The current init-once is only enabled for draw path, and it simply
generates the dev path in the init-once phase.

Bug: skia:
Change-Id: Ie9a9ef9fc453acbdeb48b06b93d578c626961e3f
Reviewed-on: https://skia-review.googlesource.com/87784
Commit-Queue: Yuqian Li <liyuqian@google.com>
Reviewed-by: Herb Derby <herb@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index ea71c8a..d45f692 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1490,13 +1490,7 @@
     test_lib("samples") {
       public_include_dirs = [ "samplecode" ]
       include_dirs = [ "experimental" ]
-      sources = samples_sources + [
-                  # Relocating these files here, so that clients don't try to build them while they're
-                  # still in active development. Clang's thread safety analysis gets tripped up by
-                  # conditional locks.
-                  "src/core/SkThreadedBMPDevice.cpp",
-                  "src/core/SkThreadedBMPDevice.h",
-                ]
+      sources = samples_sources
       deps = [
         ":experimental_sksg",
         ":experimental_svg_model",