Set is_try_builder on mac10.13_blink_rel in builders.json.

To optimize baselines properly, rebaseline-cl needs mac10.13_blink_rel
in its try bot set, because mac10.13 is the only port whose baseline
search path begins with "platform/mac".  If BaselineOptimizer doesn't
see a baseline in platform/mac, it will consolidate matching baselines
from other Mac ports under platform/mac-mac10.12, and fail to merge them
with matching baselines in platform/win into the generic path.

Bug: 812784, 774301
Change-Id: Ifda7a0051bed79a044ab516b40c0795242247b46
Reviewed-on: https://chromium-review.googlesource.com/922791
Commit-Queue: Steve Kobes <skobes@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537146}

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

@@ -70,7 +70,8 @@
     },
     "mac10.13_blink_rel": {
         "port_name": "mac-mac10.13",
-        "specifiers": ["Mac10.13", "Release"]
+        "specifiers": ["Mac10.13", "Release"],
+        "is_try_builder": true
     },
     "win7_blink_rel": {
         "port_name": "win-win7",