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

Source/core/inspector/InspectorPageAgent.cpp

Index: Source/core/inspector/InspectorPageAgent.cpp
diff --git a/Source/core/inspector/InspectorPageAgent.cpp b/Source/core/inspector/InspectorPageAgent.cpp
index 76ed62cb0835a5c149cf385b8e4591a769115b6d..84851a27322ac48ce016c8540ca6f09307f93864 100644
--- a/Source/core/inspector/InspectorPageAgent.cpp
+++ b/Source/core/inspector/InspectorPageAgent.cpp
@@ -660,6 +660,13 @@ void InspectorPageAgent::setDeviceMetricsOverride(ErrorString* errorString, int
     if (!deviceMetricsChanged(width, height, deviceScaleFactor, fitWindow, fontScaleFactor, textAutosizing))
         return;
 
+    Settings& settings = m_page->settings();
+    if (width && height && !settings.acceleratedCompositingEnabled()) {
+        if (errorString)
+            *errorString = "Compositing mode is not supported";
+        return;
+    }
+
     m_state->setLong(PageAgentState::pageAgentScreenWidthOverride, width);
     m_state->setLong(PageAgentState::pageAgentScreenHeightOverride, height);
     m_state->setDouble(PageAgentState::pageAgentDeviceScaleFactorOverride, deviceScaleFactor);
@@ -1187,6 +1194,11 @@ void InspectorPageAgent::applyEmulatedMedia(String* media)
         *media = emulatedMedia;
 }
 
+void InspectorPageAgent::canForceCompositingMode(ErrorString* errorString, bool* result)
+{
+    *result = m_page->settings().acceleratedCompositingEnabled();
+}
+
 void InspectorPageAgent::setForceCompositingMode(ErrorString* errorString, bool force)
 {
     Settings& settings = m_page->settings();