Provision to start base::SimpleTestTickClock at initial ticks

cc/scheduler/begin_frame_source_unittest.cc

Index: cc/scheduler/begin_frame_source_unittest.cc
diff --git a/cc/scheduler/begin_frame_source_unittest.cc b/cc/scheduler/begin_frame_source_unittest.cc
index 43ba4fa795f4095c3fa3cafdea84278c99abb571..778d39c12ed9fac721fedf43d44488a915fc1d72 100644
--- a/cc/scheduler/begin_frame_source_unittest.cc
+++ b/cc/scheduler/begin_frame_source_unittest.cc
@@ -188,8 +188,8 @@ class BackToBackBeginFrameSourceTest : public ::testing::Test {
   scoped_ptr<MockBeginFrameObserver> obs_;
 
   void SetUp() override {
-    now_src_.reset(new base::SimpleTestTickClock());
-    now_src_->Advance(base::TimeDelta::FromMicroseconds(1000));
+    now_src_.reset(new base::SimpleTestTickClock(
+        base::TimeTicks() + base::TimeDelta::FromMicroseconds(1000)));
     task_runner_ =
         make_scoped_refptr(new OrderedSimpleTaskRunner(now_src_.get(), false));
     task_runner_->SetRunTaskLimit(1);
@@ -355,8 +355,8 @@ class SyntheticBeginFrameSourceTest : public ::testing::Test {
   scoped_ptr<MockBeginFrameObserver> obs_;
 
   void SetUp() override {
-    now_src_.reset(new base::SimpleTestTickClock());
-    now_src_->Advance(base::TimeDelta::FromMicroseconds(1000));
+    now_src_.reset(new base::SimpleTestTickClock(
+        base::TimeTicks() + base::TimeDelta::FromMicroseconds(1000)));
     task_runner_ =
         make_scoped_refptr(new OrderedSimpleTaskRunner(now_src_.get(), false));
     source_ = TestSyntheticBeginFrameSource::Create(