DevTools: check for canForceCompositingMode before trying to emulate device metrics.

Source/devtools/front_end/inspector.js

Index: Source/devtools/front_end/inspector.js
diff --git a/Source/devtools/front_end/inspector.js b/Source/devtools/front_end/inspector.js
index cb61aed3a21b31b381e97fb7918c49ca6b293d3a..25eee55f063483792babfa364c601babe52fec8a 100644
--- a/Source/devtools/front_end/inspector.js
+++ b/Source/devtools/front_end/inspector.js
@@ -438,6 +438,9 @@ WebInspector._doLoadedDoneWithCapabilities = function()
 
     this.scriptSnippetModel = new WebInspector.ScriptSnippetModel(this.workspace);
 
+    this.overridesSupport = new WebInspector.OverridesSupport();
+    this.overridesSupport.applyInitialOverrides();
+
     new WebInspector.DebuggerScriptMapping(this.workspace, this.networkWorkspaceProvider);
     this.liveEditSupport = new WebInspector.LiveEditSupport(this.workspace);
     new WebInspector.CSSStyleSheetMapping(this.cssModel, this.workspace, this.networkWorkspaceProvider);
@@ -488,7 +491,6 @@ WebInspector._doLoadedDoneWithCapabilities = function()
         PageAgent.setShowViewportSizeOnResize(true, WebInspector.settings.showMetricsRulers.get());
     }
     showRulersChanged();
-    this.overridesSupport = new WebInspector.OverridesSupport();
 
     WebInspector.WorkerManager.loadCompleted();
     InspectorFrontendAPI.loadCompleted();