GN: take over CommandBuffer bot

As you suspected, I see nothing preventing us from building our side of things unconditionally.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2358173002
NOTREECHECKS=true

Review-Url: https://codereview.chromium.org/2358173002
diff --git a/BUILD.gn b/BUILD.gn
index 9dd881d..4501af1 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -501,6 +501,7 @@
       "src/ports/SkImageEncoder_WIC.cpp",
       "src/ports/SkImageGeneratorWIC.cpp",
       "src/ports/SkOSFile_win.cpp",
+      "src/ports/SkOSLibrary_win.cpp",
       "src/ports/SkScalerContext_win_dw.cpp",
       "src/ports/SkTLS_win.cpp",
       "src/ports/SkTypeface_win_dw.cpp",
@@ -511,6 +512,7 @@
   } else {
     sources += [
       "src/ports/SkOSFile_posix.cpp",
+      "src/ports/SkOSLibrary_posix.cpp",
       "src/ports/SkTLS_pthread.cpp",
       "src/xps/SkDocument_XPS_None.cpp",
     ]
@@ -634,6 +636,7 @@
         "tools/gpu/GrTest.cpp",
         "tools/gpu/TestContext.cpp",
         "tools/gpu/gl/GLTestContext.cpp",
+        "tools/gpu/gl/command_buffer/GLTestContext_command_buffer.cpp",
         "tools/gpu/gl/debug/DebugGLTestContext.cpp",
         "tools/gpu/gl/debug/GrBufferObj.cpp",
         "tools/gpu/gl/debug/GrFrameBufferObj.cpp",