Promote layout tests to prod on Win7

The tests are passing on CI now, so make it block the CQ.

NOTRY=true

Bug: 831847
Change-Id: I9767bb792b99cf3a47975062f6820c1fc82f8f6a
Reviewed-on: https://chromium-review.googlesource.com/1035794
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@{#565030}

testing/buildbot/chromium.win.json

@@ -2418,7 +2418,6 @@
         }
       },
       {
-        "experiment_percentage": 100,
         "isolate_name": "webkit_layout_tests_exparchive",
         "merge": {
           "args": [

testing/buildbot/test_suite_exceptions.pyl

@@ -2883,7 +2883,6 @@
       },
 
       'Win7 Tests (1)': {
-        'experiment_percentage': 100,
       },
 
       # chromium.win