[Fuchsia] Add fuchsia_x64 trybot to blink's builers.json

rebaseline_cl tool wasn't rebaselining test expectations for fuchsia
because the fuchsia try bot wasn't listed in builders.json.

Bug: 958017
Change-Id: I7968b22d3cc0a31f3e4a5083b7017af982100cae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1640796
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665669}

third_party/blink/tools/blinkpy/common/config/builders.json

@@ -64,6 +64,11 @@
         "port_name": "win-win10",
         "specifiers": ["Win10", "Debug"]
     },
+    "fuchsia_x64": {
+        "port_name": "fuchsia",
+        "specifiers": ["Fuchsia", "Release"],
+        "is_try_builder": true
+    },
     "linux-blink-rel": {
         "port_name": "linux-trusty",
         "specifiers": ["Trusty", "Release"],