Add Mac10.12 builders to list of supported Blink builders.

After this is added, Mac10.12 builders will be used when getting new baselines (rebaseline-o-matic and rebaseline-cl), and and will be checked when removing flaky expectations (update-test-expectations).

I believe that https://codereview.chromium.org/2738053002 should be committed first.

BUG=697971

Review-Url: https://codereview.chromium.org/2731403011
Cr-Commit-Position: refs/heads/master@{#455767}

third_party/WebKit/Tools/Scripts/webkitpy/common/config/builders.py

@@ -67,6 +67,10 @@ BUILDERS = {
         "port_name": "mac-retina",
         "specifiers": ['Retina', 'Release']
     },
+    "WebKit Mac10.12": {
+        "port_name": "mac-mac10.12",
+        "specifiers": ['Mac10.12', 'Release']
+    },
     "WebKit Android (Nexus4)": {
         "port_name": "android",
         "specifiers": ['Android', 'Release']
@@ -96,6 +100,11 @@ BUILDERS = {
         "specifiers": ['Retina', 'Release'],
         "is_try_builder": True,
     },
+    "mac10.12_blink_rel": {
+        "port_name": "mac-mac10.12",
+        "specifiers": ['Mac10.12', 'Release'],
+        "is_try_builder": True,
+    },
     "win7_blink_rel": {
         "port_name": "win-win7",
         "specifiers": ['Win7', 'Release'],