[Fuchsia] Add filter file for headless_browsertests to filter target

Bug: 757053
Change-Id: Iddc2c1bd3cd19f1ee2613f7c273fbcfad79426e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2231442Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Chong Gu <chonggu@google.com>
Cr-Commit-Position: refs/heads/master@{#775236}

testing/buildbot/filters/BUILD.gn

@@ -136,6 +136,7 @@ source_set("fuchsia_filters") {
     "//testing/buildbot/filters/fuchsia.blink_unittests.filter",
     "//testing/buildbot/filters/fuchsia.components_unittests.filter",
     "//testing/buildbot/filters/fuchsia.content_unittests.filter",
+    "//testing/buildbot/filters/fuchsia.headless_browsertests.filter",
     "//testing/buildbot/filters/fuchsia.mojo_unittests.filter",
     "//testing/buildbot/filters/fuchsia.net_perftests.filter",
     "//testing/buildbot/filters/fuchsia.net_unittests.filter",