Add pixel_browser_tests filter to BUILD file

This is needed for any new filter file.

Bug: chromium:958242
Change-Id: Icd95e0d04d405ac3ace5b7caf9ccb6da5a178627
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1775444
Auto-Submit: Sven Zheng <svenzheng@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691715}

testing/buildbot/filters/BUILD.gn

@@ -38,6 +38,7 @@ source_set("browser_tests_filters") {
   data = [
     "//testing/buildbot/filters/bfcache.browser_tests.filter",
     "//testing/buildbot/filters/code_coverage.browser_tests.filter",
+    "//testing/buildbot/filters/pixel_browser_tests.filter",
     "//testing/buildbot/filters/skia_renderer.browser_tests.filter",
     "//testing/buildbot/filters/webrtc_functional.browser_tests.filter",
     "//testing/buildbot/filters/webui_html_imports_polyfill_browser_tests.filter",