Adding dependency

Turns out pylib is needed to discover all the benchmarks in
benchmark_runner

Bug: 758632
Change-Id: I1b4d1106c8c32ab87b7431fa9331d67777a925a9
Reviewed-on: https://chromium-review.googlesource.com/893208Reviewed-by: Emily Hanley <eyaich@chromium.org>
Commit-Queue: Ashley Enstad <ashleymarie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532895}

tools/perf/BUILD.gn

@@ -60,5 +60,8 @@ group("perf_experimental") {
     # Field trial dependencies
     "//tools/json_comment_eater/",
     "//tools/json_to_struct/",
+
+    # For Pylib used by benchmarks
+    "//build/android/pylib/",
   ]
 }