[DevTools] Initial implementation of device modes.

Source/devtools/front_end/emulation/overrides.css

Index: Source/devtools/front_end/emulation/overrides.css
diff --git a/Source/devtools/front_end/emulation/overrides.css b/Source/devtools/front_end/emulation/overrides.css
index a8254a6db591dd0860d6ddf21684fd401c612fd0..06de2d5c32485cdfe2cca66cea13e192d564779f 100644
--- a/Source/devtools/front_end/emulation/overrides.css
+++ b/Source/devtools/front_end/emulation/overrides.css
@@ -146,6 +146,24 @@
     height: 20px;
 }
 
+.device-art .overrides-device .overrides-device-model-section select.device-select {
+    width: 180px;
+    margin-right: 6px;
+}
+
+.overrides-device .overrides-device-model-section .mode-select {
+    display: none;
+    width: 140px;
+}
+
+.overrides-device .overrides-device-model-section .mode-select.invisible {
+    visibility: hidden;
+}
+
+.device-art .overrides-device .overrides-device-model-section .mode-select {
+    display: initial;
+}
+
 /* Media tab */
 
 .overrides-media > label {