Win10 Tests x64 (debug): Fix tests

Forces layout tests to run in a debug mode.

Bug: 818832
Change-Id: I860e41169ea759e32806ba4ab45ca4e0d72ba4cd
Reviewed-on: https://chromium-review.googlesource.com/1014634Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551230}

testing/buildbot/chromium.win.json

@@ -1771,6 +1771,9 @@
         }
       },
       {
+        "args": [
+          "--debug"
+        ],
         "isolate_name": "webkit_layout_tests_exparchive",
         "merge": {
           "args": [

testing/buildbot/test_suite_exceptions.pyl

@@ -3644,6 +3644,11 @@
           ],
         },
       },
+      'Win10 Tests x64 (dbg)': {
+        'args': [
+          '--debug',
+        ],
+      },
     },
   },
   'webkit_unit_tests': {