Re-add layout tests to win7 bot

Bug: 831585
Change-Id: I3436ca3abe39a648a150d703173715f69756e508
Reviewed-on: https://chromium-review.googlesource.com/1028009
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553868}

testing/buildbot/chromium.win.json

@@ -2440,6 +2440,26 @@
           "can_use_on_swarming_builders": true
         }
       },
+      {
+        "args": [
+          "--zero-tests-executed-ok"
+        ],
+        "experiment_percentage": 100,
+        "isolate_name": "webkit_layout_tests_exparchive",
+        "merge": {
+          "args": [
+            "--verbose"
+          ],
+          "script": "//third_party/blink/tools/merge_web_test_results.py"
+        },
+        "name": "webkit_layout_tests",
+        "only_retry_failed_tests": true,
+        "results_handler": "layout tests",
+        "swarming": {
+          "can_use_on_swarming_builders": true,
+          "shards": 12
+        }
+      },
       {
         "isolate_name": "webkit_python_tests",
         "name": "webkit_python_tests",

testing/buildbot/test_suite_exceptions.pyl

@@ -3328,7 +3328,6 @@
       # chromium.win
       'Win 7 Tests x64 (1)',
       'Win10 Tests x64',
-      'Win7 Tests (1)',
     ],
     'modifications': {
       # chromium.fyi
@@ -3563,6 +3562,10 @@
         },
       },
 
+      'Win7 Tests (1)': {
+        'experiment_percentage': 100,
+      },
+
       # chromium.win
       'Win7 Tests (dbg)(1)': {
         'args': [