Enable dcheck_always_on on Win 10 FYI bot.

scripts/slave/recipes/chromium.expected/full_chromium_fyi_Chromium_Win_10.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\kill_processes.py"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "taskkill"
   },
@@ skipping 97 matching lines @@
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]\\gclient.py",
       "runhooks"
     ],
     "cwd": "[SLAVE_BUILD]",
     "env": {
-      "GYP_DEFINES": "chromium_win_pch=0 component=static_library fastbuild=1 gomadir='[BUILD]\\goma' target_arch=x64 use_goma=1",
+      "GYP_DEFINES": "chromium_win_pch=0 component=static_library dcheck_always_on=1 fastbuild=1 gomadir='[BUILD]\\goma' target_arch=x64 use_goma=1",
       "PATH": "%(PATH)s;RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]\\src\\testing\\scripts\\get_compile_targets.py",
       "--output",
@@ skipping 75 matching lines @@
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]