Move threaded BMP device to samples

Currently only referenced by SampleApp, and this squelches false positive
warnings from Clang's thread safety analysis when clients build this code.

Bug: skia:
Change-Id: I710fdc882e05fc5b80977139237028d1408f17db
Reviewed-on: https://skia-review.googlesource.com/20831
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index e3f87bc..bb5122a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1136,6 +1136,12 @@
                 "experimental/SkSetPoly3To3.cpp",
                 "experimental/SkSetPoly3To3_A.cpp",
                 "experimental/SkSetPoly3To3_D.cpp",
+
+                # 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",
               ]
     deps = [
       ":experimental_svg_model",