GN: add skia_use_mesa for OSMesa in tools.

infra/bots/recipes/swarm_compile.expected/Build-Ubuntu-GCC-x86_64-Release-Mesa.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport sys, os\npath = sys.argv[1]\nmode = int(sys.argv[2])\nif not os.path.isdir(path):\n  if os.path.exists(path):\n    print \"%s exists but is not a dir\" % path\n    sys.exit(1)\n  os.makedirs(path, mode)\n",
       "[CUSTOM_/_B_WORK]",
       "511"
     ],
     "name": "makedirs checkout_path",
@@ skipping 103 matching lines @@
     "env": {
       "BUILDTYPE": "Release",
       "CHROME_HEADLESS": "1",
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]:RECIPE_PACKAGE_REPO[depot_tools]:RECIPE_PACKAGE_REPO[depot_tools]:RECIPE_PACKAGE_REPO[depot_tools]",
       "SKIA_OUT": "[CUSTOM_/_B_WORK]/skia/out/Build-Ubuntu-GCC-x86_64-Release-Mesa"
     },
     "name": "gclient recurse (git config user.email)"
   },
   {
     "cmd": [
-      "make",
-      "most"
+      "[CUSTOM_/_B_WORK]/skia/bin/fetch-gn"
     ],
     "cwd": "[CUSTOM_/_B_WORK]/skia",
     "env": {
-      "BUILDTYPE": "Release",
-      "CHROME_HEADLESS": "1",
-      "CHROME_PATH": "[SLAVE_BUILD]/src",
-      "GYP_DEFINES": "skia_arch_type=x86_64 skia_mesa=1 skia_warnings_as_errors=1",
-      "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]:RECIPE_PACKAGE_REPO[depot_tools]:RECIPE_PACKAGE_REPO[depot_tools]:RECIPE_PACKAGE_REPO[depot_tools]",
-      "SKIA_OUT": "[CUSTOM_/_B_WORK]/skia/out/Build-Ubuntu-GCC-x86_64-Release-Mesa"
+      "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]:RECIPE_PACKAGE_REPO[depot_tools]:RECIPE_PACKAGE_REPO[depot_tools]:RECIPE_PACKAGE_REPO[depot_tools]"
     },
-    "name": "build most"
+    "name": "fetch-gn"
   },
   {
     "cmd": [
+      "gn",
+      "gen",
+      "[CUSTOM_/_B_WORK]/skia/out/Build-Ubuntu-GCC-x86_64-Release-Mesa/Release",
+      "--args=cc=\"gcc\" cxx=\"g++\" is_debug=false skia_use_mesa=true"
+    ],
+    "cwd": "[CUSTOM_/_B_WORK]/skia",
+    "env": {
+      "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]:RECIPE_PACKAGE_REPO[depot_tools]:RECIPE_PACKAGE_REPO[depot_tools]:RECIPE_PACKAGE_REPO[depot_tools]"
+    },
+    "name": "gn gen"
+  },
+  {
+    "cmd": [
+      "ninja",
+      "-C",
+      "[CUSTOM_/_B_WORK]/skia/out/Build-Ubuntu-GCC-x86_64-Release-Mesa/Release"
+    ],
+    "cwd": "[CUSTOM_/_B_WORK]/skia",
+    "env": {
+      "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]:RECIPE_PACKAGE_REPO[depot_tools]:RECIPE_PACKAGE_REPO[depot_tools]:RECIPE_PACKAGE_REPO[depot_tools]"
+    },
+    "name": "ninja"
+  },
+  {
+    "cmd": [
       "python",
       "-u",
       "import errno\nimport glob\nimport os\nimport shutil\nimport sys\n\nsrc = sys.argv[1]\ndst = sys.argv[2]\nbuild_products_whitelist = ['dm', 'dm.exe', 'get_images_from_skps', 'get_images_from_skps.exe', 'nanobench', 'nanobench.exe', '*.so', '*.dll', '*.dylib', 'skia_launcher', 'lib/*.so', 'iOSShell.app', 'iOSShell.ipa', 'visualbench', 'visualbench.exe', 'vulkan-1.dll']\n\ntry:\n  os.makedirs(dst)\nexcept OSError as e:\n  if e.errno != errno.EEXIST:\n    raise\n\nfor pattern in build_products_whitelist:\n  path = os.path.join(src, pattern)\n  for f in glob.glob(path):\n    dst_path = os.path.join(dst, os.path.relpath(f, src))\n    if not os.path.isdir(os.path.dirname(dst_path)):\n      os.makedirs(os.path.dirname(dst_path))\n    print 'Copying build product %s to %s' % (f, dst_path)\n    shutil.move(f, dst_path)\n",
       "[CUSTOM_/_B_WORK]/skia/out/Build-Ubuntu-GCC-x86_64-Release-Mesa/Release",
       "[CUSTOM_[SWARM_OUT_DIR]]/out/Release"
     ],
     "name": "copy build products",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@python.inline@import errno@@@",
       "@@@STEP_LOG_LINE@python.inline@import glob@@@",
@@ skipping 21 matching lines @@
       "@@@STEP_LOG_LINE@python.inline@    shutil.move(f, dst_path)@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]