Add --js-flags=--expose-gc flag in OilpanGCTimes test.

It enables renderers without DumpRenderTree to trigger Blink GCs.

BUG=438074, 420515

Review URL: https://codereview.chromium.org/964283002

Cr-Commit-Position: refs/heads/master@{#318888}

tools/perf/measurements/oilpan_gc_times.py

@@ -176,4 +176,5 @@ class OilpanGCTimesForInternals(_OilpanGCTimesBase):
   def CustomizeBrowserOptions(cls, options):
     # 'expose-internals-for-testing' can be enabled on content shell.
     assert 'content-shell' in options.browser_type
-    options.AppendExtraBrowserArgs('--expose-internals-for-testing')
+    options.AppendExtraBrowserArgs(['--expose-internals-for-testing',
+                                    '--js-flags=--expose-gc'])