Update some documentation for gles2_conform_support.

gpu/gles2_conform_support/README

Index: gpu/gles2_conform_support/README
diff --git a/gpu/gles2_conform_support/README b/gpu/gles2_conform_support/README
index 04305616a5ef03acf83d3e5fc1970853ce482746..d58910d5e0b6de43cad753da324b90b6b9f33784 100644
--- a/gpu/gles2_conform_support/README
+++ b/gpu/gles2_conform_support/README
@@ -1,10 +1,14 @@
 To run OpenGL ES 2.0 conformance tests, do the following:
 (These require access to Google-internal sources.)
 1. Generate build files:
-   "python build/gyp_chromium gpu/gles2_conform_support/gles2_conform_test.gyp"
-   or set "GYP_DEFINES=internal_gles2_conform_tests=1" and rebuild your
-   projects using your standard method. Example:
-   "GYP_DEFINES=internal_gles2_conform_tests=1 gclient runhooks"
+   [GYP]
+     "python build/gyp_chromium gpu/gles2_conform_support/gles2_conform_test.gyp"
+     or set "GYP_DEFINES=internal_gles2_conform_tests=1" and rebuild your
+     projects using your standard method. Example:
+     "GYP_DEFINES=internal_gles2_conform_tests=1 gclient runhooks"
+   [GN]
+     "gn args out/gn" and add the config
+     "internal_gles2_conform_tests = true"
 2. Build any of the two targets:
    - gles2_conform_test_angle which tests ANGLE
    - gles2_conform_test_native which tests command-buffer service