Change PageTest.WillStartBrowser to depend on platform instead of browser object as it would not ha…

tools/perf/benchmarks/indexeddb_perf.py

Index: tools/perf/benchmarks/indexeddb_perf.py
diff --git a/tools/perf/benchmarks/indexeddb_perf.py b/tools/perf/benchmarks/indexeddb_perf.py
index 3bc8cc8b1f6504479560e9c75c5160a6858f9c10..8db9e54b97c0acd88c1a59be89313fe4501ca26f 100644
--- a/tools/perf/benchmarks/indexeddb_perf.py
+++ b/tools/perf/benchmarks/indexeddb_perf.py
@@ -42,9 +42,9 @@ class _IndexedDbMeasurement(page_test.PageTest):
     self._power_metric = None
     self._v8_object_stats_metric = None
 
-  def WillStartBrowser(self, browser):
+  def WillStartBrowser(self, platform):
     """Initialize metrics once right before the browser has been launched."""
-    self._power_metric = power.PowerMetric(browser)
+    self._power_metric = power.PowerMetric(platform)
 
   def DidStartBrowser(self, browser):
     """Initialize metrics once right after the browser has been launched."""