Move the test runner to a higher DPI screen. (#3688)

Having the device pixel ratio of the test shell be 1.0x makes it
unlikely that we will catch errors relating to mishandling of the
device pixel ratio in the test shell.

This patch arbitrarily increases the device pixel resolution while
keeping the logical resolution the same.

shell/testing/test_runner.cc

@@ -15,8 +15,9 @@ namespace shell {
 
 TestRunner::TestRunner() : platform_view_(new PlatformViewTest()) {
   blink::ViewportMetrics metrics;
-  metrics.physical_width = 800;
-  metrics.physical_height = 600;
+  metrics.device_pixel_ratio = 3.0;
+  metrics.physical_width = 2400; // 800 at 3x resolution
+  metrics.physical_height = 1800; // 600 at 3x resolution
 
   blink::Threads::UI()->PostTask(
       [ engine = platform_view_->engine().GetWeakPtr(), metrics ] {