Enable NVPR with command buffer

Set Chromium command-line flag --enable-gl-path-rendering
before creating command buffer context. This is needed for
the command buffer to expose NVPR at the moment.

BUG=skia:2992
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1538863002

Review URL: https://codereview.chromium.org/1538863002
diff --git a/gyp/ports.gyp b/gyp/ports.gyp
index 35f5284..d685b2a 100644
--- a/gyp/ports.gyp
+++ b/gyp/ports.gyp
@@ -49,6 +49,8 @@
 
         '../src/ports/SkGlobalInitialization_default.cpp',
         '../src/ports/SkMemory_malloc.cpp',
+        '../src/ports/SkOSEnvironment.h',
+        '../src/ports/SkOSEnvironment.cpp',
         '../src/ports/SkOSFile_posix.cpp',
         '../src/ports/SkOSFile_stdio.cpp',
         '../src/ports/SkOSFile_win.cpp',