GN: gpu support in tools

If you want to try this out, the DOCS_PREVIEW link is a quickstart.

Are the GrGLCreateNativeInteface_* meant to be part of Skia-the-library
or just used by testing tools?  I've got it written here as tool-only,
but we could easily move it to libskia if it's a thing we want to ship.

I'm honestly not sure I understand how this linked before without a definition
of GrGLCreateNativeInterface().  Or are there two, one real and one no-op
default with some sort of weak linking setup?  dlopen()?

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2201233003
DOCS_PREVIEW= https://skia.org/user/quick/gn?cl=2201233003

Review-Url: https://codereview.chromium.org/2201233003

BUILD.gn

@@ -363,14 +363,20 @@ test_lib("gpu_tool_utils") {
   libs = []
 
   if (is_linux) {
-    sources += [ "tools/gpu/gl/glx/CreatePlatformGLTestContext_glx.cpp" ]
+    sources += [
+        "src/gpu/gl/glx/GrGLCreateNativeInterface_glx.cpp",
+        "tools/gpu/gl/glx/CreatePlatformGLTestContext_glx.cpp",
+    ]
     libs += [
       "GL",
       "GLU",
       "X11",
     ]
   } else if (is_mac) {
-    sources += [ "tools/gpu/gl/mac/CreatePlatformGLTestContext_mac.cpp" ]
+    sources += [
+        "src/gpu/gl/mac/GrGLCreateNativeInterface_mac.cpp",
+        "tools/gpu/gl/mac/CreatePlatformGLTestContext_mac.cpp",
+    ]
     libs += [ "OpenGL.framework" ]
   }
 }

site/user/quick/gn.md

@@ -12,7 +12,7 @@ Supported Features
 ----------
 
     * Linux, Mac
-    * Software rendering
+    * Software and GL rendering
     * libskia.a, libskia.so
     * DM, nanobench