WebUI: Roll shadycss to version 1.6.0

This improves Web UI load times with Polymer 2 by 11-14%.

Bug: 738611
Change-Id: I2bf47a8b72730d019772824f8af9359dad30b364
Reviewed-on: https://chromium-review.googlesource.com/c/1351421
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611398}

third_party/polymer/v1_0/bower.json

@@ -45,7 +45,7 @@
     "paper-tooltip": "PolymerElements/paper-tooltip#2.0.0",
     "polymer2": "Polymer/polymer#2.6.0",
     "polymer": "Polymer/polymer#1.11.2",
-    "shadycss": "webcomponents/shadycss#1.2.1",
+    "shadycss": "webcomponents/shadycss#1.6.0",
     "web-animations-js": "web-animations/web-animations-js#2.2.2"
   }
 }