Re-enable performance_browser_tests on win8

The latest comment in bug 475940, though dated, seems to indicate the
tests run just fine.  The only way to know for sure is to re-enable them
and confirm they are no longer flaky.

BUG=475940
TBR=simonhatch@chromium.org

Review URL: https://codereview.chromium.org/1553573002

Cr-Commit-Position: refs/heads/master@{#367112}

testing/buildbot/chromium.perf.json

@@ -446,6 +446,17 @@
   },
   "Win 8 Perf (2)": {
     "scripts": [
+      {
+        "args": [
+          "performance_browser_tests",
+          "--test-launcher-print-test-stdio=always",
+          "--gtest_filter=TabCapturePerformanceTest.*:CastV2PerformanceTest.*",
+          "--test-launcher-jobs=1",
+          "--enable-gpu"
+        ],
+        "name": "performance_browser_tests",
+        "script": "gtest_perf_test.py"
+      },
       {
         "args": [
           "load_library_perf_tests",