Add Site Isolation Android FYI filter file to BUILD.gn

This is a followup to https://crrev.com/c/1365982, which introduced
a new filter file for Site Isolation Android content_browsertests,
but forgot to add it to BUILD.gn.

Change-Id: I33d9626562886bd5b5b7eec051a40bc9169ed627
Reviewed-on: https://chromium-review.googlesource.com/c/1371197Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615413}

testing/buildbot/filters/BUILD.gn

@@ -68,6 +68,7 @@ source_set("content_browsertests_filters") {
     "//testing/buildbot/filters/cast-linux.content_browsertests.filter",
     "//testing/buildbot/filters/chromeos.single_process_mash.content_browsertests.filter",
     "//testing/buildbot/filters/mojo.fyi.network_content_browsertests.filter",
+    "//testing/buildbot/filters/site_isolation_android.content_browsertests.filter",
   ]
 }