wpt expectations updater: Discard empty |versions| in simplify_specifiers()

Since commit 4874fdef ("[blinkpy] Stop rebaselining on Android bots"),
android_blink_rel is no longer part of the rebaselining bots used by
wpt-importer.

However, 'android' is still part of Port.CONFIGURATION_SPECIFIER_MACROS,
which is passed as |specifier_macros| to simplify_specifiers(). And since
Android is no longer covered by trybots, |versions| in that function ends up
an empty set, which by definition means all its elements are in
|specifiers|.

In practice, this means that a test such as
external/wpt/preload/onload-event.html, which was failing on some Mac bots,
would cause simplify_specifiers() to return something like ['Android',
'Mac10.10'] and we would try to add repeated lines to TestExpectations that
look like

     crbug.com/626703 [ Android ] foo [ Failure Timeout ]
     crbug.com/626703 [ Mac10.13 ] foo [ Failure Timeout ]
     crbug.com/626703 [ Android ] foo [ Failure Timeout ]
     crbug.com/626703 [ Mac10.12 ] foo [ Failure Timeout ]

which fails our presubmit checks as expected.

Fix it by not taking empty sets into consideration when looping. This
unblocks WPT imports again.

Bug: 567947
Change-Id: Ia471cad213192fa23425d5e8eecf47457605f9ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1631663
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Commit-Queue: Robert Ma <robertma@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663740}

third_party/blink/tools/blinkpy/w3c/wpt_expectations_updater.py

@@ -433,6 +433,8 @@ class WPTExpectationsUpdater(object):
 
             # Only consider version specifiers that have corresponding try bots.
             versions = {s.lower() for s in versions if s.lower() in covered_by_try_bots}
+            if len(versions) == 0:
+                continue
             if versions <= specifiers:
                 specifiers -= versions
                 specifiers.add(macro)

third_party/blink/tools/blinkpy/w3c/wpt_expectations_updater_unittest.py

@@ -412,6 +412,16 @@ class WPTExpectationsUpdaterTest(LoggingTestCase):
         self.assertEqual(updater.simplify_specifiers(['mac10.10', 'mac10.11'], macros), ['Mac'])
         self.assertEqual(updater.simplify_specifiers(['Mac10.10', 'Mac10.11', 'Trusty', 'Win7', 'Win10'], macros), [])
 
+    def test_simplify_specifiers_port_not_tested_by_trybots(self):
+        host = self.mock_host()
+        updater = WPTExpectationsUpdater(host)
+        macros = {
+            'mac': ['Mac10.10', 'mac10.11'],
+            'win': ['win10'],
+            'foo': ['bar'],
+        }
+        self.assertEqual(updater.simplify_specifiers(['mac10.10', 'mac10.11'], macros), ['Mac'])
+
     def test_normalized_specifiers_with_skipped_test(self):
         host = self.mock_host()
         expectations_path = '/test.checkout/wtests/NeverFixTests'