buildbot: Enable merging Mac output.

testing/buildbot/chromium.fyi.json

Index: testing/buildbot/chromium.fyi.json
diff --git a/testing/buildbot/chromium.fyi.json b/testing/buildbot/chromium.fyi.json
index ddc1b5065c9f67003951950c6330389d5f4079d9..043083805b962ca40b5906ab3cbb5520183f8fd8 100644
--- a/testing/buildbot/chromium.fyi.json
+++ b/testing/buildbot/chromium.fyi.json
@@ -12065,7 +12065,23 @@
     "isolated_scripts": [
       {
         "isolate_name": "webkit_layout_tests_exparchive",
+        "merge": {
+          "args": [
+            "--verbose",
+            "--results-json-override-with-build-property",
+            "build_number",
+            "buildnumber",
+            "--results-json-override-with-build-property",
+            "builder_name",
+            "buildername",
+            "--results-json-override-with-build-property",
+            "chromium_revision",
+            "got_revision_cp"
+          ],
+          "script": "//third_party/WebKit/Tools/Scripts/merge-layout-test-results"
+        },
         "name": "webkit_layout_tests",
+        "results_handler": "layout tests",
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [