Add flag to use virtual contexts to WebGraphicsContext3D::Attributes

Source/Platform/chromium/public/WebGraphicsContext3D.h

Index: Source/Platform/chromium/public/WebGraphicsContext3D.h
diff --git a/Source/Platform/chromium/public/WebGraphicsContext3D.h b/Source/Platform/chromium/public/WebGraphicsContext3D.h
index f515c2762d17897368685ecc51a5d96423d0b314..6f9f262fc8e80e58983174608882dd665cf19950 100644
--- a/Source/Platform/chromium/public/WebGraphicsContext3D.h
+++ b/Source/Platform/chromium/public/WebGraphicsContext3D.h
@@ -94,6 +94,7 @@ public:
             , shareResources(true)
             , preferDiscreteGPU(false)
             , noAutomaticFlushes(false)
+            , useVirtualContext(false)
         {
         }
 
@@ -107,6 +108,7 @@ public:
         bool shareResources;
         bool preferDiscreteGPU;
         bool noAutomaticFlushes;
+        bool useVirtualContext;
         // FIXME: ideally this would be a WebURL, but it is currently not
         // possible to pass a WebURL by value across the WebKit API boundary.
         // See https://bugs.webkit.org/show_bug.cgi?id=103793#c13 .