Run GC after each memory test page

Before this CL, no GC was run in between test, so setting --pageset-repeat > 1
lead to increased memory usage per test run, reporting inaccurate results.
This CL forces V8 GC (and attached Oilpan GC) after each test run for stable result.

More background:
Blink used to trigger V8 major GC after page unload, but this was changed to
less aggressive GC since r181187, suspected as source of this "leak".

BUG=465997

Review URL: https://codereview.chromium.org/1027133002

Cr-Commit-Position: refs/heads/master@{#322318}

tools/perf/measurements/memory.py

@@ -24,6 +24,9 @@ class Memory(page_test.PageTest):
     self._memory_metric.Start(page, tab)
     self._power_metric.Start(page, tab)
 
+  def CleanUpAfterPage(self, page, tab):
+    tab.CollectGarbage()
+
   def CustomizeBrowserOptions(self, options):
     memory.MemoryMetric.CustomizeBrowserOptions(options)
     # Since this is a memory benchmark, we want to sample memory histograms at