GN: add skia_use_mesa for OSMesa in tools.

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

Index: infra/bots/recipes/swarm_compile.expected/Build-Ubuntu-GCC-x86_64-Release-Mesa.json
diff --git a/infra/bots/recipes/swarm_compile.expected/Build-Ubuntu-GCC-x86_64-Release-Mesa.json b/infra/bots/recipes/swarm_compile.expected/Build-Ubuntu-GCC-x86_64-Release-Mesa.json
index 03253fbef4f350ba1cad323a0c376daf80c0894a..59fe37fec08080cac2ad817fa0eee005153dc75c 100644
--- a/infra/bots/recipes/swarm_compile.expected/Build-Ubuntu-GCC-x86_64-Release-Mesa.json
+++ b/infra/bots/recipes/swarm_compile.expected/Build-Ubuntu-GCC-x86_64-Release-Mesa.json
@@ -121,19 +121,38 @@
   },
   {
     "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": "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": "build most"
+    "name": "ninja"
   },
   {
     "cmd": [