Revert "Drop the layout tests to experimental on Mac10.13 Tests."

This reverts commit 7bfc1624.

Reason for revert: tests appear to have mostly recovered; unclear why.

Original change's description:
> Drop the layout tests to experimental on Mac10.13 Tests.
> 
> TBR=dpranke@chromium.org
> 
> Bug: 868180
> Change-Id: Ic9cb48f82954f102f60dd3e1b9c29bcca209f3bb
> Reviewed-on: https://chromium-review.googlesource.com/1152566
> Reviewed-by: John Budorick <jbudorick@chromium.org>
> Commit-Queue: John Budorick <jbudorick@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#578626}

TBR=dpranke@chromium.org,jbudorick@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 868180
Change-Id: Ibb217f63bb5b834565fafac4cb7ad15543ba70bb
Reviewed-on: https://chromium-review.googlesource.com/1155614Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579286}

testing/buildbot/chromium.mac.json

@@ -2865,7 +2865,6 @@
         }
       },
       {
-        "experiment_percentage": 100,
         "isolate_name": "webkit_layout_tests_exparchive",
         "merge": {
           "args": [

testing/buildbot/test_suite_exceptions.pyl

@@ -1719,8 +1719,6 @@
           ],
           'shards': 12,
         },
-
-        'experiment_percentage': 100,  # crbug.com/868180
       },
       'Mac10.13 Tests (dbg)': {
         'args': [