Use CSS pixels for scrolling tests.

telemetry/telemetry/internal/actions/scroll.js

Index: telemetry/telemetry/internal/actions/scroll.js
diff --git a/telemetry/telemetry/internal/actions/scroll.js b/telemetry/telemetry/internal/actions/scroll.js
index 1a295d05fca083781984c7d27193d27d3a6bc368..d23cd63984dab41518992ece4a3ca5b7e0d68059 100644
--- a/telemetry/telemetry/internal/actions/scroll.js
+++ b/telemetry/telemetry/internal/actions/scroll.js
@@ -124,10 +124,14 @@
                             this.getScrollDistance_());
 
     var rect = __GestureCommon_GetBoundingVisibleRect(this.options_.element_);
+    // TODO(bccheng): workaround crbug/599656
+    // Use device-independent pixel value for the x and y positions
     var start_left =
-        rect.left + rect.width * this.options_.left_start_ratio_;
+        rect.left + (rect.width / window.devicePixelRatio) *
+        this.options_.left_start_ratio_;
     var start_top =
-        rect.top + rect.height * this.options_.top_start_ratio_;
+        rect.top + (rect.height / window.devicePixelRatio) *
+        this.options_.top_start_ratio_;
     chrome.gpuBenchmarking.smoothScrollBy(
         distance, this.onGestureComplete_.bind(this), start_left, start_top,
         this.options_.gesture_source_type_, this.options_.direction_,