Fix UI Devtools CSS Agent Unit Tests

Modified unit tests for changes to CSSAgent.

Bug: 977214
Change-Id: I5710a35cfa19955fa3ceccb11c0d5c098fdc498e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1668310
Commit-Queue: Kristyn Hamasaki <khamasaki@google.com>
Reviewed-by: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676935}

components/ui_devtools/BUILD.gn

@@ -138,8 +138,7 @@ source_set("unit_tests") {
   }
 
   sources = [
-    #TODO(lgrey,weili): fix and enable css agent tests.
-    #"css_agent_unittest.cc",
+    "css_agent_unittest.cc",
     "devtools_server_unittest.cc",
   ]