DevTools: fix screenshots on mac headless while emulating.

Bug: 741969
Change-Id: Id80b178aee043babd25a533e21fc25118a446d77
Reviewed-on: https://chromium-review.googlesource.com/572424Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Pavel Feldman <pfeldman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486940}

content/browser/devtools/protocol/page_handler.cc

@@ -377,8 +377,10 @@ void PageHandler::CaptureScreenshot(
     if (!modified_params.view_size.width)
       emulated_view_size = original_view_size;
 
-    double dpfactor = modified_params.device_scale_factor /
-                      widget_host->GetView()->current_device_scale_factor();
+    ScreenInfo screen_info;
+    widget_host->GetScreenInfo(&screen_info);
+    double dpfactor =
+        modified_params.device_scale_factor / screen_info.device_scale_factor;
     modified_params.scale = dpfactor;
     modified_params.view_size.width = emulated_view_size.width();
     modified_params.view_size.height = emulated_view_size.height();